and as for being aliens in the universe. . we are one galaxy, the milky way. outside of our galaxy, with our current highest powered telescopes, we can see ten other galaxies. and of course, there are millions and millions of galaxies out there. .
in order for aliens to come here, they would need to travel at the speed of light. (there are no life forms in the milky way galaxy cept humans). itd take, from the nearest galaxy to ours, 4 light years to get here. (a light year is number of years it takes traveling at the speed of light to get from point a to point b). taking that into consideration, aliens would need to go at at least a tenth the speed of light to get to earth and back to their planet in their lifetime. (assuming that their lifetime is about 80 years). and dont say "travel at those speeds isnt possible". 100 years ago we thought flight was not possible. before that, computers and most of todays technology was unthinkable.