“He recalls to mind his various temporary states in days gone by – one birth, or two or three or four or five births, 10 or 20, 30 or 50, a 100 or a 1,000 or a 100,000 births, through many cycles of cosmic contraction and cosmic expansion . . . Now there comes a time, when sooner or later, after the lapse of a long, long period of contraction, this world-system passes away. And when this happens beings have mostly been re-born in the World of Radiance, and there they dwell made of mind, feeding on joy, radiating light from themselves, traversing the air, dwelling in glory; and thus they remain for a long, long period of time. Now there comes also a time, friends, when sooner or later, this universe begins to re-evolve by expansion.” (Brahmajala Sutta, Digha Nikaya)
The Buddha also says that within these world systems are "thousands of suns, thousands of moons, thousands of continents" (Anguttara Nikaya 1. 227). He also speaks of the "the black, gloomy regions of darkness between the world systems, where the light of our moon and sun, powerful and majestic though they are, cannot reach" (Majjhima Nikaya 3. 124), suggesting that these galaxies are great distances away from each other in space.
In 2013 it was reported that "Many more planets are thus out there, zipping undetected around their parent stars. Indeed, a team of researchers estimated last year that every Milky Way star hosts, on average, 1.6 worlds — meaning that our galaxy perhaps harbors 160 billion planets." 
- "Number of Confirmed Alien Planets nears 1,000"