I am interested in science (particular how the universe works) and so I watch the Discovery Channel. Does this Stack Exchange site cover questions about the universe or does it only cover questions about Earth?
Earth science only covers a subset of the universe.
For the earth, earth science covers processes from the planetary core to the extent of its atmosphere. As the processes that occur on earth are shared by other planets, our scope also includes other planets.
The space between planets is not part of our scope. Likewise physics that are unrelated to earth science processes is not part of our scope (e.g. quantum field theory belongs on physics.stackexchange.com, not here).
Another Stack Exchange site to consider is Astronomy.SE which is specifically about astronomy and astrophysics. This site is in beta (like here) and has a group of astronomers there happy to answer questions (some of them are fast).