  If dark matter is ultralight, the number density of dark matter is very high
and the techniques of zero-temperature field theory are no longer valid. The
dark matter number density modifies the vacuum giving it a non-negligible
particle occupation number. For fermionic dark matter, this occupation number
can be no larger than one. However, in the case of bosons the occupation number
is unbounded. If there is a large occupation number, the Bose enhancement needs
to be taken into consideration for any process involving particles which
interact with the dark matter. Because the occupation number scales inversely
with the dark matter mass, this effect is most prominent for ultralight dark
matter. In fact, the Bose enhancement effect from the background is so
significant for ultralight dark matter that, if dark matter is a dark photon,
the correction to the anomalous magnetic moment is larger than experimental
uncertainties for a mixing parameter of order $10^{-16}$ and a dark photon mass
of order $10^{-20}$ eV. Furthermore, the constraint on the mixing parameter
scales linearly with the dark photon mass and so new significant constraints
can be placed on the dark matter mass all the way up to $10^{-14}$ eV. Future
experiments measuring $g-2$ will probe even smaller gauge mixing parameters.
