It has obvious visible effects, like the way it bends light from distant galaxies. (gizmodo.com)
But a few years later, when Edwin Hubble's analysis of light from distant galaxies confirmed the universe's expansion, Einstein gave in. (sciencenews.org)
Light from distant galaxies passing through those regions also gets warped, making the galaxies appear streaked and smeared in telescope images. (newscientist.com)