If we can go from cave-dwelling hominids to an internet-using and robot-building society in 200,000 years, what could an alien race achieve in 10 billion years?

That's more than enough time for a civilization to develop sophisticated rockets — possibly faster-than-light travel, wormhole technology, or some other kind of cosmic shortcut that would allow them to rapidly colonize the galaxy and beyond.

The Kardashev Scale, created by astrophysicist Nikolai Kardashev, is helpful when considering such technological advancement by a developing civilization. It has three types:

1. A Type-I civilization has figured out how to harness all the energy on its planet. Humans are getting close to achieving this, but that's just the first tier.
2. Type-II civilizations are so intelligent they've figured out how to harness all the energy of their own star — an incomprehensibly larger amount of energy than what's available on one puny planet.
3. That's nothing compared to the Type-III civilizations, though. Those have harnessed all the energy available in their galaxy.

You can group the best explanations for the paradox into two distinct categories: one in which aliens don't exist, and we're completely alone in the universe, and one in which aliens do exist, but for some reason we haven't made contact.

Don't despair. There are other solutions to the Fermi Paradox:

http://pandawhale.com/post/67764/ten-solutions-to-the-fermi-paradox

By the way, the scale of our universe is mind boggling.

Even on the clearest, darkest night far from city lights, you can see only about 1% of the Milky Way galaxy's 100 billion to 400 billion stars.

Here's the real trip though: For every star in the Milky Way, there's a unique galaxy drifting through the universe, each with its own billions of stars, and approximately one planet orbiting each of those stars. That's billions and billions and billions of worlds.

And yet decades' worth of missions by Search for Extraterrestrial Intelligence (SETI), an organization which listens for signs of life in space, have come up completely empty handed. Every. Single. One.

Physicist Enrico Fermi is famous for posing the natural question that follows: Where is everybody? The scale of the universe and basic math tell us alien life must be common, yet there's no evidence for it.

A whole lot more about the Fermi Paradox:

http://pandawhale.com/post/62683/the-fermi-paradox-where-are-all-the-aliens-by-kurzgesagt