Originally Posted by NCC-73515
I thought the idea of expansion comes from the redshift...
It does, but there are two causes of redshift: movement of the target away from the viewer and filtering, mainly by the one atom of H1 per cubic centimeter in space. They account for H1 filtering accurately, because the amount of H1 in space can be measured with radioastronomy. But the H2 molecule is not detectable in the same way or with any other technology currently available, except where there's a nearby extreme heat source to light it up. That's how it was first observed about 14 years ago with the European Space Agency's Infrared Space Observatory.
A detailed explanation is here: