We discover that the mass of dark matter particles m DM is imprinted in phase correlations of the cosmic density field more significantly than in the two-point correlation. In particular, phase correlations trace m DM out to scales about five times larger than the two-point correlation. This result relies on a new estimator ℓ(r) of pure phase information in Fourier space, which can be interpreted as a parameter-free and scale-invariant tracer of filament-like structure. Based on simulated density fields, we show how m DM can, in principle, be measured using ℓ(r), given a suitably reconstructed density field. © 2013. The American Astronomical Society. All rights reserved..