It's fairly long but you can get the theory from this first clip. The theory is that cosmic rays, both from the sun and the galaxy create aerosols that allow the formation of low clouds. Low clouds reflect more of the sun's energy back to space and the earth cools. A correlation between the earth's galactic position, and the climate over eons provides anecdotal evidence in support for the theory. Experimental results simulating cosmic rays to see how they effect aerosol creation supports the theory. It took these scientists years to get their study published because it didn't fit the narrative of AWG. Here's the money quote (from part 2)
"the whole climate community really hated the idea that sun should have a major impact on climate; that was seen as a disaster."
This is a fantastic video.