  Neutrons travel along straight lines in free space, but only survive for a
distance which depends on their energy. Thus, detecting neutrons in space in
principle provides directional and distance information. Apart from secondary
neutrons produced by cosmic-ray interactions in the Earth atmosphere, which are
the dominant background, direct neutron emission is caused by solar flares,
with clear time correlation with X-rays, which can be measured from few tens
MeV up to few GeV. There is no detectable astrophysical source up to the PeV
scale, when neutrons coming from supernova remnants may reach the Earth before
decaying. In addition, ultra high energy neutrons are the most plausible
explanation for the measured anisotropy of cosmic-ray showers produced in the
atmosphere above $10^{18}$ eV. From the GeV to the PeV scale, the expected
neutron flux is very low and not too different from the antiproton flux, as the
same cosmic-ray collisions with the interstellar medium which can produce
antiprotons can also produce neutrons and antineutrons. This background flux of
cosmic-ray neutrons is very low and has not yet been detected. Measuring the
neutron energy spectrum in space is a very effective way of searching for
decays of exotic particles. For example, dark matter could consist of Weakly
Interacting Massive Particles (WIMPs), which may annihilate into final states
with particle and antiparticle pairs. Consequently, a number of indirect WIMP
searches are being carried on, focusing on positron and antiproton spectra.
However, no experiment is presently foreseen to look for "bumps" in the neutron
energy spectrum, which is virtually background free. Here we consider the
implications of a measurement of the neutron energy spectrum in astronomy and
astrophysics and list the interesting energy regions in the search for WIMPs.
