Constraints on Axionlike Dark Matter with Masses Down to 10^{-23}  eV/c^{2}.

We analyzed a 6.7-yr span of data from a rotating torsion-pendulum containing approximate to 10(23) polarized electrons to search for the "wind" arising from ultralight, axionlike dark matter with masses between 10(-23) and 10(-18) eV/c(2). Over much of this range we set a 95% confidence limit F-a/C-e > 2 x 10(15) eV on the axionlike decay constant.