Clifford D. Simak 
Cosmic Engineers 

"Upon you and you alone must rest the fate of the universe. You are the only ones to save it." Thus spoke the mysterious Cosmic Engineers to a small group of human beings on the rim of the solar system. Somewhere out there in the vastness of the galaxies lurked the greatest challenge they would ever face – the catastrophic fury of the Hellhounds of Space. Promptly, courageously the earthlings boarded their galactic spaceships and journeyed out far beyond uncharted stars, plunging into dangers too awful even to contemplate.