What do you think will be the next great discovery in astronomy? 

One of the lovely things about working in astronomy is the number of
surprising results that keep rolling in. So to answer this question is
quite dangerous, because I'd be proved wrong by tomorrow. But nevermind.
One of the great mysteries of the universe at the moment is to do with
material we call "dark matter". It seems that the stuff that we can see
only accounts for perhaps 5 percent of the whole universe. And the other
95 percent is something that we have yet to understand. It's important to
understand it and it's important to know how much there is of it because
that determines what the future of the universe will be. And right now
there is a lot of work going on trying to understand what it is that makes
up this dark matter. And I guess we'll have the answer (or maybe answers
because it may be more than one thing). I think we'll know the answer or
answers in the next 5-10 years. And that will be a very important
discovery in astronomy.