The nearest large spiral neighbour of the Milky Way…

The nearest large spiral neighbour of the Milky Way, lying about two and a half million light years away. On clear, dark nights it is visible to the naked eye as a faint smudge, making it one of the most distant objects observable without a telescope. It contains roughly a trillion stars. Moving towards us, it may eventually collide and merge with our own star system over billions of years.

The answer in 15 letters