In planetary physics, the Lagrange points are points in a two-body system where the gravitational attraction from the two bodies are equal. There are five such points; L1-L3 are unstable while L4 and L5 are stable. The Earth-Moon L4 and L5 points have attracted interest as possible sites for space colonies.
But what's already there? There has only been one space mission through L4 and L5 (the Hiten spacecraft), which found no increase in the amount of interplanetary dust. (That was the only instrument it had.) It seems to me that all sorts of interplanetary flotsam should end up there, from dust to fairly good-sized rocks. Some of them should be visible (via light or radar) from the ground. It would also seem to be a good place to collect samples of interplanetary flotsam; much easier than going all the way out to the asteroid belt.
How big is the Universe? What's its mass? I don't mean the *visible* universe -- I mean the whole thing. According to current cosmology, immediately after the "big bang", the universe went through a period of "inflation", where space itself expanded far faster than the speed of light. This spread out all of the "wrinkles" in space-time that should have been generated by the Big Bang, so that we see, for example, the cosmic background radiation as being very smooth in all directions.
However, this means that there are parts of the universe that we can't see -- they're "over the horizon". Inflation pushed them to a distance that their light simply can't get to us.
According to the Wikipedia article above, estimates of the size of the total Universe range from 250 times the size of the visible universe to 3×1023 times the size of the visible universe. I'd think that this would be a critical parameter for any "theory of everything". With estimates differing by 21 orders of magnitude, there's obviously room for research.