Although the Universe is a large place, and all the stuff in it may seem just flung everywhere higgledy-piggledy, there's rather more structure than we can see.
According to our models of the Universe, and mounting evidence, filaments of dark matter connect massive objects such as galaxies and galaxy clusters in a vast, cosmic web.
It's along these filaments that hydrogen flows, feeding into the galaxies, but they're not so easy to see - among all the brightly glowing stars and galaxies and galactic nuclei, the faint emission from diffuse hydrogen in intergalactic space is hard to see, never mind map.
Image: Hydrogen filaments (in blue). (Roland Bacon/David Mary/ESO/NASA)
