  Dark matter (DM) particles in the Universe accumulate in neutron stars (NSs)
through their interactions with ordinary matter. It has been known that their
annihilation inside the NS core causes late-time heating, with which the
surface temperature becomes a constant value of $T_s \simeq (2-3) \times 10^3$
K for the NS age $t \gtrsim 10^{6-7}$ years. This conclusion is, however, drawn
based on the assumption that the beta equilibrium is maintained in NSs
throughout their life, which turns out to be invalid for rotating pulsars. The
slowdown in the pulsar rotation drives the NS matter out of beta equilibrium,
and the resultant imbalance in chemical potentials induces late-time heating,
dubbed as rotochemical heating. This effect can heat a NS up to $T_s \simeq
10^6$ K for $t \simeq 10^{6-7}$ years. In fact, recent observations found
several old NSs whose surface temperature is much higher than the prediction of
the standard cooling scenario and is consistent with the rotochemical heating.
Motivated by these observations, in this letter, we reevaluate the significance
of the DM heating in NSs, including the effect of the rotochemical heating. We
then show that the signature of DM heating can still be detected in old
ordinary pulsars, while it is concealed by the rotochemical heating for old
millisecond pulsars. To confirm the evidence for the DM heating, however, it is
necessary to improve our knowledge on nucleon pairing gaps as well as to
evaluate the initial period of the pulsars accurately. In any cases, a
discovery of a very cold NS can give a robust constraint on the DM heating, and
thus on DM models. To demonstrate this, as an example, we also discuss the case
that the DM is the neutral component of an electroweak multiplet, and show that
an observation of a NS with $T_s \lesssim 10^3$ K imposes a stringent
constraint on such a DM candidate.
