WATCH: Neil deGrasse Tyson explains why humanity hasn't made first contact with aliens yet

During the question-and-answer session after a lecture in Denver on Thursday, acclaimed astrophysicist Neil deGrasse Tyson addressed the question of why the human race has never had contact with alien species.

He began by explaining the Fermi Paradox.

"[Enrico Fermi] said that the universe has been around a really long time, and technological evolution, when it happens, happens fast. If there are [advanced] aliens in the galaxy, they should have been here by now. Because if they live approximately as long as we do, they can send colonies out to other star systems, set up base camps, and then [those base camps] send out other colonies."

"So one grows to ten, grows to a hundred, grows to a thousand," Tyson explained. "And you can grow the number of colonies exponentially in the world very quickly, so where are they?"

"I have three answers to that question. One is, they've already visited, but they happened to land during Comic-Con, and nobody noticed them," he joked.

"Another hypothesis is that if you are of a culture that wants to colonize another planet, another star system, that says something about you. You're not happy with just one planet, you want more."

"So you go, and you claim this new planet," Tyson continued. "This keeps going, but if everybody who's doing that has that mentality, you reach the point where you start running out of planets. And then what happens? War."

"It may be that the very urge to colonize star systems is the force that prevents it from colonizing the entire galaxy -- because they put themselves out of business. The land-grabbing that goes on is inconsistent with what is actually required to create a harmonious system of colonized planets across the galaxy."

"A third possibility," he concluded, "and I'm all in for this one, is that we have been visited -- on days other than when Comic-Con was in progress -- but they looked around and concluded that there was no intelligent life on Earth and left forever."

Watch his entire answer via John Rubinetti on YouTube below.