Lyman-alpha forest

Yeah.

Specifically, I'm thinking about the Lyman-alpha forest, the data we get from observing light passing through those really distant clouds of neutral hydrogen.

Right, and that is the exact frontier of the debate right now.

It's a very hot topic.

Okay, let's unpack that.

So the Lyman-alpha forest allows us to measure the distribution of matter on relatively small cosmic scales.

Traditional cold dark matter, the heavy particle model, it predicts that matter clumps together at all scales from massive superclusters all the way down to very small localized clumps.

Because heavy particles have no problem clumping.