The following is a list of some of the key unanswered questions in Modern Physics:
1. Can General Relativity and Quantum Mechanics be reconciled? This is also known as the Problem of Quantum Gravity
2. What is time? Does it even exist?
3. What happened before the Big Bang?
4. How did the great constants of physics come to be?
5. What particles comprise Dark Matter? Does it even exist or should we modify Newton's Law of Gravitation to make sense of large mass motion in the galaxy? How does Dark Matter influence supersymmmetry?
6. What really is Dark Energy? How does it truly influence the universe?
7. Do Multiverses exist and if so how do they influence our understanding of all that there is?
8. What is reality? Can this be reconciled within the world of Quantum mechanics? Is there a level of formal structure behind Quantum foam?
9. Why is the speed of light constant in a vacuum?
10. Does the Higgs Boson particle exist? How can we find it? Will it really tell us what the concept of mass is?
11. Do strings exist?
12. Can we unify the Four Fundamental forces of nature? - gravity being the hold out.
13. Why is there more matter in the universe than anti-matter? - Baryon symmetry problem
14. Why did the universe have such low entropy in the past? - so-called Arrow of Time
15. How is neutrino mass generated? What is the mechanism?
16. How does Electroweak Symmetry breaking occur? What gives mass to W and Z bosons?
17. What accounts for the missing spin in protons? - quarks only account for 12% - Could it be the Gluons?
18. How can the mathematics of Quantum Chromodynamics (QCD) give rise to the nucleus and its constituents?
19. Why do Cosmic Rays close to Earth have such high energy considering than their don't appear to be any significant sources?
20. What type of physics can explain the Tetraneutron or the Pentaquark?
21. How many dimensions are there? What is a dimension?