Space is a big, lonely place, so it"s nice to know we have neighbors. The pictured Andromeda galaxy is our Milky Way"s closest neighbor—right next door at 2.5 million light-years away. Our cordial relationship with Andromeda goes back about 10 billion years to when both galaxies were still forming. But trouble is brewing: Andromeda is on a collision course with the Milky Way, due for impact in 4 billion years. This neighborhood"s about to get rough…