1. Black Holes
    It's believed they lie at the center of every large galaxy. And they're born after the death of a star. Black holes. Objects with such a high level of density, that not even light can escape. They break multiple laws of earthly physics, and shed light on the most complicated contradictions of space-time. Example? The closer you get to a black hole, the slower you experience the passing of time (due to the gravitational force). What feels like seconds for you, could be decades for us...
  2. Dark Matter
    Basically, somebody applied the equated strength of gravity to the rotation speed of our galaxies. The conclusion? There wasn't enough gravity to hold it together. In theory, galaxies should spiral out of control and cease to exist. That's when we discovered there's something else holding them (and now we know, the rest of the universe) together: Dark Matter. It makes up 90% of the universe, and we have no idea what it is...yet.
  3. Expansion
    Yes, more space is being added to space. Nobody knows from where. Nobody knows what it's expanding into.
  4. The Edge of Space
    Because the expansion of the universe got an early start on human space exploration (by 13 billion years), there's no possible way we'd ever find the "edge" of space. It's too far away, and expanding too quickly.