The universe is a darker place than most people see. Literally. The person who proved this was Gunnar Malmquist, who realized that we are blind to the dark side of the universe.
When you look up at the sky at night you know that you aren't seeing all the stars. You're only seeing the brightest ones. If you had a telescope, or more sensitive instruments, you could probably see more. That doesn't entirely solve the problem, though. Depending on the sensitivity and range of the telescope you can see more bright stars that are farther away, and a few dimmer stars that are close up. But there's still a distance at which stars at a brightness level that you could see close up dwindle into darkness. Gunnar Malmquist, a famous astronomer, realized this back in the 1920s. You will see, and count, only stars above a certain level of brightness.
This wouldn't be so much of a problem if, when we are looking out into space, we were looking into the present. Actually, we're looking into the past. We can't take for granted that the compensations we make for our neck of the woods count when we look at galaxies millions of years into the past. One major example is dwarf galaxies. These are clusters of stars that aren't very bright. When they're close, we can see how many of them there are - compared to other galaxies. When we look far away, and see far fewer dwarf galaxies, what does that tell us? It could mean that we can't see the dwarf galaxies. It could mean that, as we're looking millions of years into the past, they haven't formed yet. It could mean that they should be formed, but some unknown variable is keeping them from forming. Although there are various ways of compensating for the Malmquist Bias, there's no doubt that it reminds us that we live in a world with an unseen dark side.
Image: X-ray: NASA/CXC/Huntingdon Institute for X-ray Astronomy/G. Garmire; Optical: ESO/VLT