I don't think we need to concern ourselves with aliens from other galaxies. Even within our own, the interstellar distances are prohibitively large. Mutiply that by several orders of magnitude and add the fact that galaxies speed away from each other and you quickly arrive at a conclusion that an intergalactic invasion is not something you need to lose sleep over. Dark matter? It may be a convenient shortcut to explain certain aspects of the universe expansion but to me, it reeks too much of "horror vaccui", the medieval idea that nature abhors empty space and wants to fill it. That's why when you suck on a tube whose other end is immersed in water, the water fills the tube, right? Wrong, as it turned out. It is the weight of the surrounding air that pushes the water into the tube. I suspect that dark matter and dark energy will find a similar, more natural explanation one day. I wish science were more humble and honestly admitted, "I don't know yet, but I may one day", rather that coming up with esoteric conceps like dark matter. Though I admit that "dark matter" sounds catchier.