Enrico Fermi, an American nuclear physicist, realised that any civilisation with a modest amount of rocket technology and the urge to explore and colonise could inhabit the entire Galaxy within a few million years. A million years is not a long time at all in cosmic terms. Our sun has been a stable star and the Solar System has been in existence for about 4,500 times as long. In fact, we should have been colonised several times and the planet should be littered by evidence of war between alien races, or at least coexistence
The question arose then, "why has not Earth been contacted or colonised by aliens?"
The answer seemed to be that their were not any aliens, or at least no aliens intelligent enough to find the means to leave their home planet. But we know from studies of life on Earth that intelligence is extremely useful to survival and helps to propagate the species. It seems very likely that life, once created, must evolve into an intelligent form. Once intelligent, the urge to explore and conquer must surely follow.
Possible resolutions to the paradox exist, all of them unappetising. Maybe we are the only intelligent life in our Galaxy. Maybe the lifetime of a civilisation does not typically mean it can colonise the Galaxy. The human race has been around for about 100,000 years. Some calculations indicate it could die out within 50,000 years because people, once they reach a certain level of prosperity, do not make enough babies. Instead, they watch TV, grow fat, and die young and childless.