Astronomers working with the Hubble telescope now believe that the space between galaxies not not empty, but is rather filled with a vast, diffuse web of baryonic matter – mainly hydrogen. This “cosmic web” may account for around half of the missing mass of the universe, but does not preclude the need for the existence of more massive clumps of dark matter. (The visible universe only accounts for around 4% of the expected total mass.) Story here and here.