What is the biggest question in astrophysics right now...?

Ultra High Energy Cosmic Rays - Cosmic rays have been measured with energies as high as 3 x 10^20 eV. How do subatomic particles obtain these enormous energies?

Energy loss due to interactions with the microwave background should restrict the origin of these particles to within 50 Mpc. Yet no sources are seen within this volume. Because these particles are extremely rare, determining what these particles are and where they come from will push current technological limits.

Aloha

dark matter and high energy cosmic rays are certainly two of the biggest topics in astrophysics to date. however; a phenomenon that has the ability to destroy a galaxy, and seems to be happening at random throughout the universe has got to be at the top of the list. the gamma ray burst gets my vote.

The nature and distribution of Dark Matter is probably the biggest question right now. A good second question is exactly how galaxies originally formed (was it top-down or bottom-up?).

Dark Matter is a good one. Another good one would be the ongoing mission to find a relationship between gravity and the other forces of nature, the Grand Unified Theory, as it were.

I'm not directly involved in this field, so just an opinion. I would vote for the need to identify dark matter as my numero uno.