Martin Rees is sitting in the Master’s Lodge of Trinity College, Cambridge, with a laptop balanced on his knee. ‘I want to show you this,’ he says, tapping the keys with long, neat fingernails. Two red swirls appear on either side of the screen, gliding towards each other. When they meet it’s messy, like two ripe tomatoes smashing together in mid-air. We are watching one of the most violent events in the universe. ‘That’s two galaxies colliding,’ he explains.In five billion years our own galaxy may be one of the exploding fruit. That’s when the Milky Way is predicted to crash into Andromeda, creating an intergalactic splatter known as ‘Milkomeda’. But like many things in our universe, it’s far from certain. Few people are better equipped to follow this cosmic food fight than Rees.