Where is the roughly 30-40% of the universe's ordinary (baryonic) matter that Big Bang nucleosynthesis and cosmic microwave background measurements predict should exist, but which could not be directly observed in stars, galaxies, and cold gas in the more recent universe?
Why do electrons confined to two dimensions in a strong magnetic field condense into exotic liquid states carrying fractionally charged, sometimes non-abelian, quasiparticles?
Can a laser-like device be built that emits coherent gamma rays (a 'graser'), using nuclear rather than atomic transitions, and how far can achievable laser intensities be pushed?
How far up the periodic table can atomic nuclei be pushed by fusing lighter nuclei together, and does a predicted 'island of stability' of unusually long-lived superheavy isotopes actually exist?
Can the masses of protons, neutrons and other hadrons be computed from the underlying quark-gluon theory (QCD), and can the free quark-gluon plasma believed to have filled the early universe be created and studied in the laboratory?
Does the electroweak theory unifying electromagnetism and the weak nuclear force correctly predict a Higgs boson responsible for elementary-particle masses, and can that particle actually be found?
Where and how strongly does nature violate the combined charge-parity (CP) symmetry, and can the observed amount explain why the universe is made of matter rather than antimatter?
What is causing the expansion of the universe to accelerate (dark energy, or a cosmological constant), and did the universe undergo a brief phase of exponential inflationary expansion in its first fraction of a second?
What is the exact state of matter inside a neutron star (its equation of state), and what physical mechanism actually revives a massive star's collapsing core into a supernova explosion?
How do the supermassive black holes that power quasars and sit at galaxy centers form and grow, and how do galaxies themselves assemble over cosmic time?
When a drop or stone hits a liquid surface fast enough, why does the splash form a crown of regularly-spaced spikes, and how does the crown height depend on impact speed and drop size?
Why can basilisk lizards run across open water without sinking, and could a human do the same thing at ordinary Earth gravity?
As you gyrate your hips, why does a hula hoop rise and stay suspended around your waist instead of falling to the ground, and what shape of body is required for this to work at all?
A moving bicycle with no rider steering it can recover from being pushed sideways and keep rolling upright by itself: what physical effect makes it self-stabilize?
If a ship's propeller is driven by a wind-turbine-like air motor connected to the wheels or propeller, can such a vehicle move faster than the wind, including directly against it?
Given a snow-covered mountain slope of a certain height and angle, roughly how fast will an avalanche be moving by the time it reaches the bottom, and how far will the debris travel?
If you send a large enough electrical current pulse through a thin wire to make it vaporize, what is the maximum magnetic field achievable near the wire before it disintegrates, and what ultimately limits the field strength reachable this way?
Neutrons, like electrons, have a wavelength that could in principle be used for microscopy: how does the achievable resolving power of a neutron microscope compare to an electron microscope, and what fundamentally limits it?
If two artificial satellites moving in opposite directions along intersecting orbits collide directly, what happens to the debris, and does this pose any longer-term hazard to other satellites?
If the Sun suddenly vanished, would Earth feel the loss of its gravitational pull at the same instant the light disappeared, or would there be a delay -- and can this be measured?