I have been listening to Sean Carroll's Mindscape podcast on the Crisis in Physics. I was put off at first because it is over 4 hours long but I have listened in installments. It's brilliant. I am learning so much and am going to try to explain to myself what I learned. He explains evidence for dark matter. One piece is from Silk Damping which comes under Diffusion Damping. This is how the differences in density in the early universe were smoothed out. Photons were released and free to travel. They interact with normal matter and took protons and electrons with them. This evened out the density but it oscillated until it settled down. If I have understood it correctly, it is damped by gravity. Here is a normal damping graph from mechanics with exponential decay in amplitude.
Here is the power spectrum from the Cosmic Microwave Background. The issue that Joseph Silk explained is why these two peaks have the same amplitude. The photons cannot interact with dark matter so the dark matter has no part in the outward expansion. However, on the rebound dark matter can exert a gravitational pull, making the peak bigger than expected. That's what I understood the podcast to be saying!
