The Andromeda Galaxy — M31 — is the most distant object visible to the unaided human eye, at 2.537 million light-years from us. The light you see when you find it tonight left the galaxy when our ancestors were still figuring out fire.
It is a smear of pale grey on a dark night, roughly the size of six full moons end-to-end, though most observers only see the brighter central core at first. Patience and averted vision — looking slightly to one side — reveal the wider disc.
Andromeda and the Milky Way are on a collision course, due to merge in approximately 4.5 billion years. The two galaxies will not so much crash as interpenetrate, their stars largely missing one another despite the chaos of gas and gravity.