How Late can the Dark Matter form in our universe?
  We put constraints on the epoch of dark matter formation for a class of non-WIMP (Weakly Interacting Massive Particle) dark matter candidates. These models allow a fraction of Cold Dark Matter (CDM) to be formed between the epoch of Big Bang Nucleosynthesis (BBN) and the matter radiation equality. We show that for such models the matter power spectra might get strong suppression even on scales that could be probed by linear perturbation theory at low redshifts. Unlike the case of Warm Dark Matter (WDM), where the mass of the dark matter particle controls the suppression scale, in Late Forming Dark Matter (LFDM) scenario, it is the redshift of the dark matter formation which determines the form of the matter power spectra. We use the Sloan Digital Sky Survey (SDSS) galaxy clustering data and the linear matter power spectrum reconstructed from the Lyman-$\alpha$ data to find the latest epoch of the dark matter formation in our universe. If all the observed dark matter is late forming, we find lower bounds on the redshift of dark matter formation $z_f > 1.08 \times 10^5 $ at 99.73 $\%$ C.L from the SDSS data and $z_f > 9 \times 10^5$, at the same C.L, from the Lyman-$\alpha$ data. If onlya fraction of the dark matter is late forming then we find tentative evidence of the presence of LFDM from the Lyman-$\alpha$ data. Upcoming data from SDSS-III/BOSS (Baryon Oscillation Spectroscopic Survey) will allow us to explore this issue in more detail. 
Abir Sarkar, Subinoy Das and Shiv K. Sethi
astro-ph.CO