I’m not alone, not by a longshot, when I say I love the idea of space exploration and possible alien cultures.

Look at some of the staples of pop culture — Star Trek and Star Wars, for example. These classic sci-fi stories have given us thrilling images of new worlds and aliens of all sorts. We can add one of my favorites, SyFy’s brutally cancelled Farscape, and one of the newest movie franchises, Guardians of the Galaxy (highly recommended, btw).

In all of these, the universe is thick with life. There are countless races of intelligent—and not-so-intelligent—life forms, numbering perhaps trillions of individuals.

But, as far as we know, we are utterly alone in the universe, and we don’t know why.

As scientists discover solar system after solar system, with planets in the habitable zone, it’s dawning on us that our planet is not unique. And the logical assumption would be, if Earth is not unique, then we are not either. Surely if life evolved on Earth, over millions of years, to produce a species that is capable of traveling into space, then at least one of these other countless planets would have evolved similar life as well.

But where are they? Set aside the assumptions we’re making, such as that we would even be able to recognize alien life at all. If other species developed interstellar travel, wouldn’t they have found us by now? Wouldn’t their presence have long been known?

One of my favorite sci-fi books, Revelation Space by Alastair Reynolds, tackled this question. Reynolds had an intriguing, though wholly fictional answer. (SPOILER WARNING) In Revelation Space, there were indeed alien races spread throughout the universe. But they went to war and ended up wiping each other out. To avoid any such catastrophe, a super-entity was established that would snuff out any civilization that got too big for its britches. How would it to that? Simply by waiting patiently for a curious species to contact it, and then exterminating it.

Another theory for the lack of alien life is more simple. Maybe complex and intelligent life is extremely rare — so rare that we’re it. It’s called the Great Filter theory. Several nearly impossible steps had to be overcome for us to be here.

–the creation of molecules that can reproduce

–the creation of simple single-celled life

–the creation of complex, multi-celled life

And that’s not even getting into such things as the rise of intelligent life capable of traveling into space, while also avoiding threats such as asteroid strikes, nuclear war, radiation bursts from space, and so on.

So, maybe we really are alone, and there is no Star Trek style federation waiting to greet us.

The good news? Maybe we’ve already overcome the biggest hurdles to interstellar life, and the universe is ours for the taking.