Astrophysics is a pretty neat field. Even so, when we start discussing the really big picture - the structure of the Universe, galaxy clusters, and so on - it starts to sound a lot like botany. We know what we see, and we can classify and describe it, but not in any sort of elegant way. There's some fundamental understanding that we still lack. On the one hand, this can be a little disappointing. On the other, it means that we have things left to learn, and my guess is that we'll be figuring bits and pieces out in the next couple of decades.
Even just within our galaxy, we can't fully explain the stellar orbits we observe. Star systems orbiting the center of the galaxy behave as if there's much more mass than we can see. That mass is called dark matter, and it makes up a huge fraction of the total mass of the Milky Way. Astrophysicists have watched the orbits of nearby stars to determine the distribution of dark matter, and particle physicists are conducting all sorts of searches for this mysterious dark matter, which doesn't emit light and only interacts with regular matter via gravity and the weak nuclear force.
And matters only get worse as we look at larger-scale structures. The 2011 Nobel Prize in physics was awarded to three astrophysicists "for the discovery of the accelerating expansion of the Universe through observations of distant supernovae." As far as we know, the Universe started in a big bang and has been expanding ever since. The bizarre thing is that it seems like the expansion should be slowing down, since all the matter is gravitationally attracted to all the other matter, but instead it's speeding up! We call the source of the extra energy to increase the expansion rate dark energy. It seems to be some sort of property of spacetime, but we have no idea what's really causing it.
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