MenoSaurus by S J House

Rex is racing to master the secrets of time and space, in order to defeat the Mantoid menace. He is shocked to learn there is only one way to wipe out the enemy. In order to save the Earth, he must travel to the end of time and exterminate the dark energy that gives the Mantoids their power. But as dark energy is the glue that holds everything together, how can he do it without destroying the universe itself?