The Great Filter
The most-common end result of a species or a planetary civilization that failed to work together towards starflight / celestial expansion and subsequent colonization, is complete and total biological destruction through the use of their own technological weaponry.
The future of human cosmic exploration is going to be human beings mostly finding the remnants of dead civilizations. Thousands upon thousands, and maybe even millions of civilizations, all taken out by their own weaponry. The Milky Way galaxy is largely a vacant parking lot; the species that evolved here, or at least ‘grew up’, are mostly long gone.
The reality of space exploration is such: there are millions upon millions, billions upon billions, perhaps even trillions upon quadrillions of functioning alien civilizations. This universe is a big fucking place, and it’s not the only universe around. Not by a long shot. But the living civilizations are, by a long fucking stretch, overwhelmed by societies that simply stopped overnight.
Human beings venturing out into outer space need to prepare themselves for the realities of spaceflight, but especially extraterrestrial exploration. You will find horrible things. Fully-functioning societies, rendered suddenly inert, overnight. You will find architecture in full working order, with most things still in one piece. But, for these dead civilizations, you won’t find any people. Perhaps not even their corpses.
The toxins and weapons these people used to destroy themselves might not even harm human beings. That’s the good news. The bad news is that the base impulses, the ‘instincts’, if you will, that allowed most species to survive in their infancies, will come to try to destroy the same species as it tries to reach the stars. The anger; the jealousy; the envy; the covetousness; the greed; the overt, and covert avarice. These are your enemies. These are the things that will kill you, before you reach the stars.
Don’t let them.