It can be both observed and detected, it can be mapped even. The detection is a pretty indirect one right now though. There's the usual stuff about galaxy rota

It can be both observed and detected, it can be mapped even. The detection is a pretty indirect one right now though. There's the usual stuff about galaxy rotation curves, there's the http://en.wikipedia.org/wiki/Bullet_Cluster http://en.wikipedia.org/wiki/Bullet_Cluster which people consider to be more of a smoking gun, but then there's also evidence from http://en.wikipedia.org/wiki/Baryon_acoustic_oscillations http://en.wikipedia.org/wiki/Baryon_acoustic_oscillations visible in the CMB. That last one's slightly over my head, but apparently cosmologists find it extremely convincing, especially when combined with the other evidence. The wiki page talks about dark energy, but the subject has significance for dark matter too. Let me see if I can find the relevant plot... At the end of the day it's just another weakly interacting particle. Last time we discovered one of those was the neutrino in the 50s, it's not an outlandish thing to think about. EDIT: here's the BAO stuff: http://ned.ipac.caltech.edu/level5/Sept09/Einasto/Einasto6.html http://ned.ipac.caltech.edu/level5/Sept09/Einasto/Einasto6.h...