A gram of nucleons is Avagadro's number, 6e23, of particles. A nucleon has a collision cross section of about 1e-26 cm^2. So a gram of nucleons has a cross section of 0.006 cm^2. If dark matter is to be at best weakly interacting, it had better have a cross section way less. -- Gene From: Warren D Smith <warren.wds@gmail.com> To: math-fun@mailman.xmission.com Sent: Friday, March 27, 2015 1:19 PM Subject: [math-fun] Dark matter exists? http://arxiv.org/abs/1503.07675 claims they tested some colliding galaxy clusters and found the gas got moved, but the mass got moved by a different amount than the gas. Their techniques for measuring this seem remarkably sensitive (if we believe them). This "proves" dark matter exists [claimed p-level 10^(-14) or so]. This contrasts with earlier "proofs" which involved the rotation curves of galaxies. Two proofs are more convincing than one, and alternate "new physics" explanations (different from assuming dark matter exists, that is) now may have more trouble surviving. They then go further and estimate the interaction cross-section of dark matter particles with other dark (or non-dark) matter particles. They claim it is too small for them to detect, less than 0.47 square-cm per gram. If this can be brought below 0.1 this may actually disprove the existence of DM (!?) they say, because they say other observations of DM suggest it must self-interact. So this whole line looks very promising, at least on its face.