  We suggest that the dark matter in the universe has quantum entanglement if
the dark matter is a Bose-Einstein condensation of ultra-light scalar
particles. In this theory, any two regions of a galaxy are quantum entangled
due to the quantum nature of the condensate. We calculate the entanglement
entropy of a typical galactic halo, which turns out to be at least
$O(ln(M/m))$, where $M$ is the mass of the halo and $m$ is the mass of a dark
matter particle. The entanglement can be inferred from the rotation curves of
the galaxy or the interference patterns of the dark matter density.
