Something that baffles scientists today is a strange situation called the Fermi Paradox, named after Italian physicist Enrico Fermi.
The basic conundrum is that there's an incredibly high probability that alien life forms not only exist in the universe, but nearby Earth. The reason for this statement is the radically large number of solar systems in our galaxy alone. With so many stars in the observable universe, billions are similar to our Sun. The likelihood that many of these stars have Earth-like planets is therefore quite high. Assuming Earth is a typical planet, intelligent life must have developed on many of these planets. Our planet has existed for about 4.5 billion years in a 14 billion year old universe, so there should have been plenty of time for countless organic lifeforms to develop space travel and begin exploring our galaxy, since humans have come thus far in only 200,000 years. Finally, with rough estimates based on current hypothesis for interstellar travel (which may in fact be very slow and inefficient) the Milky Way Galaxy could be traversed in only about a million years, and totally colonized in about two million.
So, scientists wonder, where are all the aliens? Why, if life in our galaxy has had so many chances to exist, do we have such little evidence of extraterrestrials? Well, there are several different hypotheses.
One idea concerns filters. This idea states that life has many difficult to pass barriers which make its existence incredibly difficult. We've passed some already, such as the still undiscovered process through which life originates, mutually assured destruction, and extinction events. Perhaps the universe was actually incredibly hostile and dangerous for any life until only recently, making humans some of the first ever. And, there are great filters in our future as well, such as irreversible climate change. Maybe there's some impassible filter we don't know of, and won't for a long time, that no life form has yet to defeat. Plenty of people have already assumed that nuclear bombs and the Large Hadron Collider would destroy the Earth, maybe someday they'll be right.
There's also the idea that other life forms are preventing this interaction. Maybe some incredibly advanced life form from far away has advanced enough that they can control the entire galaxy, and they don't want other life forms to advance to the point where they pose a threat. Maybe they physically prevent interaction in order to stop the spread of ideas, and prevent any further development. Or, perhaps they act as a filter themselves, and annihilate and race that begins to get too far.
Or, maybe we're actually just alone. We could be the first life ever to exist, the only, and the last once we eventually kick the bucket. Any way it works out, scientists still don't really have an answer to the Fermi Paradox, and with good reason. This question is a very confusing, scary, and difficult one to answer. So, for now, all we know is that either there's no evidence of life on Earth, or the government took it.