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.
-Phenomena lacking clear 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?
What is the source of the observed accelerated expansion of the universe? What is the nature of the dark energy driving this acceleration?
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?
Entropy (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?
Are these glowing, floating objects real? How can they be explained?
Why is there far more matter than antimatter in the universe?
Black hole structure
Does a black hole have an internal structure, and if so, what is its nature?
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?
Why doesn't the zero-point energy of vacuum cause a large cosmological constant? What cancels it out? Is a non-total cancellation of the cosmological constant responsible for the observed accelerated expansion (deSitter phase) of the Universe?
What is dark matter? Is it related to supersymmetry? Do the phenomena attributed to dark matter point not to some form of matter but actually to an extension of gravity?
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
Can fundamental constants be predicted by a more complete theory? Do "fundamental physical constants" vary over time?
Gamma ray bursts (short duration)
What is the nature of these extraordinarily energetic astronomical objects that last less than two seconds?
What is the responsible mechanism that causes certain materials to exhibit superconductivity at temperatures much higher than around 50 kelvins?
How far away are the Pleiades, exactly?
Island of stability
What is the largest theoretically possible stable atom?
Why does the Minkowski metric describe the vacuum?
What is the mechanism responsible for generating neutrino masses? Is the neutrino its own antiparticle?
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 possesses 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?
Also it would be nice if someone could come up with some cool theories.