The notion of life on other planets, and space travel, all start to excede our imaginations quite quickly, once we apply them.
It's more than possible that alens exist, but they may not have physical form, and be undetectable to humans.
It's also possible that future generations will overcome all the limitations that currently prevent us from exploring space at any distance.
If you look at the speed and intricacy of scientific innovations of even the last fifty years, and I am using one now to communicate this to the world at the push of a button, it is perfectly feasible that space travel and the exploiration of the universe, is simply a matter of time.