E.E. Smith 
Triplanetary [EPUB ebook] 

Two thousand million or so years ago two galaxies were colliding; or, rather, were passing through each other. A couple of hundreds of millions of years either way do not matter, since at least that much time was required for the inter-passage. At about that same time—within the same plus-or-minus ten percent margin of error, it is believed—practically all of the suns of both those galaxies became possessed of planets.
There is much evidence to support the belief that it was not merely a coincidence that so many planets came into being at about the same time as the galactic inter-passage. Another school of thought holds that it was pure coincidence; that all suns have planets as naturally and as inevitably as cats have kittens.
Be that as it may, Arisian records are clear upon the point that before the two galaxies began to coalesce, there were never more than three solar systems present in either; and usually only one. Thus, when the sun of the planet upon which their race originated grew old and cool, the Arisians were hard put to it to preserve their culture, since they had to work against time in solving the engineering problems associated with moving a planet from an older to a younger sun.
Since nothing material was destroyed when the Eddorians were forced into the next plane of existence, their historical records also have become available. Those records—folios and tapes and playable discs of platinum alloy, resistant indefinitely even to Eddore’s noxious atmosphere—agree with those of the Arisians upon this point. Immediately before the Coalescence began there was one, and only one, planetary solar system in the Second Galaxy; and, until the advent of Eddore, the Second Galaxy was entirely devoid of intelligent life.
Thus for millions upon untold millions of years the two races, each the sole intelligent life of a galaxy, perhaps of an entire space-time continuum, remained completely in ignorance of each other. Both were already ancient at the time of the Coalescence. The only other respect in which the two were similar, however, was in the possession of minds of power…