May be the scientific view of the world looks as a stable and finished system. That is not the case. There are still many open questions in this conception of the world and also fundamental things are not clear yet. Openness becomes thus a virtue of this conception of the world. Every once in a while something in science changes and obvious contradictions are a stimulus for advancement.
- The more we learn about the world, and the deeper our learning, the more conscious, specific, and articulate will be our knowledge of what we do not know, our knowledge of our ignorance. For this, indeed, is the main source of our ignorance — the fact that our knowledge can be only finite, while our ignorance must necessarily be infinite.
- Conjectures and Refutations: The Growth of Scientific Knowledge Karl Popper (1963)
This is a list of some of the unsolved problems in physics . Some of these problems are theoretical, meaning that existing theories seem incapable of explaining some observed phenomenon or experimental result. Others are experimental, meaning that there is a difficulty in creating an experiment to test a proposed theory or investigate a phenomenon in greater detail. Lastly, some even border on the pseudo-sciences, e.g. the fringes of science which are widely discredited by today's community of physicists, but may some day show promise or conclusive evidence.
Phenomena or established occurrences in search of a scientific explanation
- Accretion disc jets
- Why do the accretion discs surrounding certain astronomical objects, such as the nuclei of active galaxies, emit relativistic jets along their polar axes?
- Accelerating universe
- Why is the expansion of the universe accelerating, as we have observed? What is the nature of the dark energy driving this acceleration? If it is due to a cosmological constant, why is the constant so small, yet non-zero? Why isn't it huge, as predicted by most quantum field theories, or zero for some yet unknown symmetry reason? What is the ultimate fate of the universe?
- Amorphous solids
- What is the nature of the transition between a fluid or regular solid and a glassy phase? What are the physical processes giving rise to the general properties of glasses?
- Arrow of time
- Why did the universe have such low entropy in the past, resulting in the distinction between past and future and the second law of thermodynamics? Why does time flow in one direction at all, on macroscopic scales, when there does not seem to be an arrow of time on the scale of fundamental interactions?
- Ball lightning
- Are these glowing, floating objects real? How can they be explained?
- Baryon asymmetry
- Why is there far more matter than antimatter in the universe?
- Cold fusion
- What is the theoretical explanation for the apparent production of excess heat and helium in palladium metal when it is saturated with deuterium?
- Corona heating problem
- Why is the Sun's Corona (atmosphere layer) so much hotter than the Sun's surface?
- Cosmological constant
- Why doesn't the zero-point energy of vacuum cause a large cosmological constant? What cancels it out?
- Why does the cosmological constant have the value that it has, is it zero and is it really constant?
- Dark matter
- What is dark matter? How is it generated? Is it related to supersymmetry? Do the phenomena attributed to dark matter really flow from some kind of matter at all, or are those phenomena in fact an indication of flawed physical laws? 
- Electroweak symmetry breaking
- What is the mechanism responsible for breaking the electroweak gauge symmetry, giving mass to the W and Z? Is it the simple Higgs mechanism of the Standard Model?
- Fundamental physical constants
- Why do we observe these values, and not others? Have the values of the so called "fundamental physical constants" varied at all over time?
- Gamma ray bursts (short duration)
- What is the nature of these extraordinarily energetic astronomical objects that last less than two seconds?
- High-temperature superconductors
- Why do certain materials exhibit superconductivity at temperatures much higher than around 50 kelvins?
- What causes anything to have mass?
- Neutrino mass
- What is the mechanism responsible for generating neutrino masses? Is the neutrino its own antiparticle?
- Pioneer anomaly
- What causes the apparent residual sunward acceleration of the Pioneer spacecraft?
- What causes the emission of short bursts of light from imploding bubbles in a liquid when excited by sound?
- Is it possible to make a theoretical model to describe the behavior of a turbulent flow (in particular, its internal structures)?
- Ultra-high-energy cosmic ray
- Why is it that some cosmic rays appear to possess energies that are impossibly high (the so called Oh-My-God particle), given that there are no sufficiently energetic cosmic ray sources near the Earth? Why is it that (apparently) some cosmic rays emitted by distant sources have energies above the Greisen-Zatsepin-Kuzmin limit?
- Universe asymmetry
- What are the origins of asymmetries in general in the Universe?
Theoretical ideas in search of experimental evidence
- Is the Peccei-Quinn theory (i.e. mechanism) the solution to the strong CP problem? What are the properties of the predicted axion?
- Cosmic inflation
- Is the theory of cosmic inflation correct, and if so, what are the details of this epoch? What is the hypothetical inflation field giving rise to inflation?
- Extra dimensions
- Does Nature have more than four spacetime dimensions?
- Is it possible to go faster than the speed of light? Is it possible to transmit information faster than the speed of light?
- Is our universe filled with gravitational radiation from the big bang? From astrophysical sources, such as inspiralling neutron stars? What can this tell us about quantum gravity and general relativity? Does gravity behave as predicted at very small distance scales?
- Do physical phenomena irrevocably destroy information about their prior states?
- Magnetic monopoles
- Are there any particles that carry "magnetic charge", and if so, why are they so difficult to detect?
- Why is there matter at all? What was here before the Big Bang? How did the four fundamental forces appear?
- Proton decay
- Many theories beyond the Standard Model predict proton decay. Do protons decay? If so, then what is their half-life?
- Quantum chromodynamics (QCD) in the non-perturbative regime
- The equations of QCD remain unsolved at energy scales relevant for describing atomic nuclei. How does QCD give rise to the physics of nuclei and nuclear constituents?
- Quantum gravity
- How can the theory of quantum mechanics be merged with the theory of general relativity to produce a so-called "theory of everything"? Is string theory the correct step on the road to quantum gravity, or a blind alley? Is there any way to extract experimental information about the nature of physics at the Planck scale?
- Quantum mechanics in the correspondence limit
- Is there a preferred interpretation of quantum mechanics? How does the quantum description of reality, which includes elements such as the superposition of states and wavefunction collapse, give rise to the reality we perceive?
- Size of the universe
- Is there anything beyond the limits of the theoretical boundaries of the observable universe?
- Standard Model Higgs mechanism
- Does the Standard Model Higgs particle exist? More generally, does inertial mass have a basis or mechanism separate from gravitational mass?
- Is supersymmetry a symmetry of Nature? If so, what is the mechanism of supersymmetry breaking?
- Does Nature make use of strong dynamics in breaking electroweak symmetry?
Problems solved recently
- Solar neutrino problem (2002)
- Resolved by a new understanding of neutrino physics, requiring a modification of the Standard Model of particle physics - specifically, neutrino oscillation.
- Quasars (1980s)
- The nature of quasars was not understood for decades. They are now accepted as a type of active galaxy where the enormous energy output results from matter falling into a massive black hole in the center of the galaxy.
- Long duration Gamma ray bursts (2003)
- Long-duration bursts are associated with the deaths of massive stars in a specific kind of supernova-like event commonly referred to as a collapsar.
- Open Questions in Physics Usenet Physics FAQ, 2006, Open Questions, Cosmology and Astrophysics item 5
- Open Questions item 4
- Physics Problems for the Next Millennium University of Michigan Strings 2000 Conference
- 13 things that do not make sense newscientistspace, 19 March 2005, Michael Brooks, item 5
- Sellwood and Kosowsky, "Does Dark Matter Exist?" 
- Open Questions, Particle Physics, item 6
- Open Questions, Particle Physics, item 12
- Open Questions, Cosmology and Astrophysics, item 11
- Open Questions, Condensed Matter and Nonlinear Dynamics, item 2
- Open Questions, Particle Physics, item 13
- newscientistspace item 8
- Open Questions, Condensed Matter and Nonlinear Dynamics
- Open Questions, Cosmology and Astrophysics, item 12
- newscientistspace item 3
- Physics News Update A weekly physics news bulletin hosted by the American Institute of Physics.
- Open Questions in Physics
- New Scientist: 13 things that do not make sense.
- Science Magazine: the most compelling puzzles and questions facing scientists today.
- List of links to unsolved problems in physics, prizes and research.
- Dark Matter - one problem solved
- Ideas Based On What We’d Like To Achieve
Some unsolved problems in biology include:
- How did it start? Is life a cosmic phenomenon? Are the conditions necessary for the origin of life narrow or broad? Did life start on this planet or was there an extraterrestrial intervention (for example a meteor from another planet)? Is immortality possible?
- DNA / Genome
- What are all the functions of the DNA? Other than the structural genes, which is the simpler part of the system? What is the complete structure and function of the proteome proteins expressed by a cell or organ at a particular time and under specific conditions? What is the complete function of the regulator genes? The building block of life may be a precursor to a generation of electronic devices and computers, but what are the electronic properties of DNA? Does Junk DNA function as molecular garbage?
- Consciousness / Self
- What is it? Why is it necessary for many animals (especially mammals) to dream? What are the inheritable characteristics of intelligence?
- How do animals possess long-range navigation and migration abilities? How was the homing ability developed? How can some animals detect earthquake precursors? What are the effects of natural electric fields?
- Viruses / Immune system
- What are the signs of current or past infection to discover where Ebola hides between human outbreaks? What is the origin of antibody diversity? What is the relationship between the immune system and the brain?
- Viking's methane
- The Viking lander detected emissions of carbon-14-containing methane, which indicates the possible presence of life on Mars.