Lightyear is a measure of distance, not time. It is the distance that light travels during one Earth year ( The speed of light is 300,000km/s, which means that this is a very, tremendously, amazingly huge distance of over 9.5 trillion km). Fact source...
Galaxies can collide. In fact, our own milky way is due to a collision with Andromeda galaxy. Not to worry though, this is going to happen in over 4 billion years from now, and even when it happens, galactic distances are so vast that the chances of a star or a planet...
