Italian physicist Enrico Fermi asked an interesting question about life in cosmos many years ago: “Where are they?”; i.e., if there is intelligent life elsewhere in our galaxy, why have those beings not revealed themselves to us? After all, even with sub-luminal speeds it will “only” take a few million years for an intrepid culture to colonize large parts of the Milky Way. This is known as Fermi’s Paradox.
However, a counter-question is: “Why do it if one’s home system suffices.” The dreams of interstellar colonization arose in a century when Earth’s population grew with a tremendous speed. The forecasts for humanity’s future were dire. But now in the 21st century, it looks like Earth will reach a population plateau in the foreseeable future so our planet will be capable of sustaining mankind for a long time. The solar system is full of mineral resources that soon will be within reach for commercial exploitation. So if there is no way of travelling faster than light, it seems likely that many advanced civilizations would stay at home: orbital habitats, a partially terraformed Mars and other man-made abodes should provide room enough for our descendants. And the Sun will supply them with all the energy they need for millions of years.
Crossing the interstellar void, on the other hand, takes generations and consumes incredible amounts of resources for uncertain gains. And the planets people could find around alien stars will most likely be inhospitable to our species: the movie Interstellar comes to mind.
Fredrick Jenet and Teviet Creighton also speculate on that issue here — link >>>