I have studied astronomy for a few years now and am very passionate about it, more-so than Japan believe it or not. So in order to get the ball rolling, feel free to ask me whatever questions you have for space and I'll get you an answer to it or at least a theory or two about it.
The nearest galaxy to our own (Milkyway) is dependent on what you would classify as a galaxy such as dwarfs, satellites and magnetic clouds. However assuming on what you mean, the Andromeda galaxy which is about 2.5milion light years away from Earth. To imagine how far this is, imagine traveling the speed of light which is 299,792,458 meters per second (a good geeky number to remember) it would take you 2.5 million years to get from here to there.
That is why it is impossible by using conventional means to travel from system to system because it would take easily millions of years to get anywhere if even possible. This doesn't mean it's impossible to travel to distant galaxies within minutes but it is impossible with our current technology.
You asked about why Mars cannot support life however I will twist that around a little by saying that it most likely does or at least has in the past. Mars used to be like Earth, to which extent we don't really know for sure but it would be a lot like the desert, somewhat grassy, and plentiful in water. You can still see where Oceans used to be on Mars by those massive "craters". It is suggested that life used to live at a micro-bacterial level especially in hot springs that are also found on earth.
The reason that Mars doesn't support life anymore is because it is now desolate and unable to provide one of the key factors for life as we know it. Water is what gives power to all beings on earth, however on other planets or systems it could be entirely possible for beings to live by sulfuric acid or take shape of a cloud. We can only compare with what we know, and unfortunately it's not much when you compare earth to the billions upon billions of galaxies that exist in our view.
When Mars used to be wet, it was able to stay that way because of the warm core in the center, however that core wasn't strong enough to keep a permanent "heart" to the planet and eventually became so small that all life if any expired from the cold. It is possible that life still exists underneath the surface, and perhaps even in hot springs that has not yet been discovered. Unfortunately, these life forms would not be intelligent and be smaller than what you could see unless they were like algae.
Is it possible to restore Mars to the way it was before and make it a second earth? In theory, this is entirely possible and one of the ways is to pollute the atmosphere so much that it would cause a greenhouse effect and melt the ice caps which would then cause rain and carbon dioxide. With our current technology if we were able to go to Mars without any issues, we would be able to restore the planet within 100-years, green grass, blue oceans, and trees. However it may take up to a thousand years before the planet is fully restored. Ironic isn't it? Pollution destroys what we know but it restores what we haven't seen. However many suggest that this method wouldn't work and that we would have to find alternative methods to restore the planet. At any rate, Mars is going to eventually get restored because Earth is exceeding its ability to feed its habitats.
I hope this answers your question.
