Observed quasar light as a function of wavelength shows absorptions due to neutral hydrogen “clouds” along the line-of-sight. This “Lyman-
forest” has a power spectrum that has a cut-off. We study this cut-off. This cut-off may be due to warm dark matter free-streaming in the early universe, or to neutral hydrogen thermal velocity Doppler shift, or both. It is difficult to distinguish between these alternatives. If warm dark matter free-streaming dominates, we obtain the following density power spectrum cut-off wavevector:
Mpc？1. This is a measure of the dark matter “warmness”. This result is in agreement with four independent published measurements, and in disagreement with published limits.
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