Light year - is a unit of distance, not time (as some sci-fi shows have sometimes erroneously used it). It's the distance light travels in a vacuum in a year; that is, 5.88 trillion miles or 9.46 trillion kilometers.
Theory, hypothesis - when people think about the word hypothesis, they think only of an idea that is to be tested in a science class. And when people use the word theory, they use it synonymously with hypothesis, which is interchangeable with idea. In the scientific community, however, the two terms are vastly different and can't be interchanged.
In science, a hypothesis is an idea - indeed something to be tested. However, a theory is not just an idea. A theory can be thought of as an hypothesis that has graduated: it's been tested numerous times, yielding the same results (within an acceptable margin of error), and is taken by the scientific community to be something close to fact.
The beauty of science is that it's constantly changing - as we learn more about the universe, we can add to or subtract from an idea. It doesn't necessarily mean the principle constructs of a particular theory is wrong; just that our understanding of it was still incomplete.
The main problem lies in the fact that the general public and the scientific community don't use the word theory in the same way. I cringe every time I hear the term "it's just a theory" used in a sneering sort of way.
And on a "lighter" note (heh):
"He is happiest, be he king or peasant, who finds peace in his home." ~ Johann von Goethe
