Guys I'm sorry but as far as colonizing the universe and why haven't aliens done it.. so far the best explanation we have with the most data and testing is the rare earth hypothesis. There is obviousely much more to the evolution of intelligent life than the number of stars in a galaxy or as the drake equation has it laid out. Chances are most life that could have had a chance was made extinct just as most things are extinct here for example. I think its possible somebody is out there but I bet its WAYY out there. Until we get some probes to other stars and other classes of stars we will have nothing other than our own little corner to base our calculations on for the odds.. and those odds aren't very good so far.