The chargeless, weakly interacting neutrinos are ideal astronomical messengers as they travel through space without scattering, absorption or deflection. But this weak interaction also makes them notoriously difficult to detect, leading to neutrino observatories requiring large-scale detectors. A few years ago, the IceCube experiment discovered neutrinos originating beyond the Sun with energies bracketed by those of the highest energy gamma rays and cosmic rays. I discuss how these high-energy neutrinos can be detected and what they can tell us about the origins of cosmic rays and about dark matter. Neutrinos from deep space can be used as astronomical messengers, providing clues about the origin of cosmic rays or dark matter. The IceCube experiment is leading the way in neutrino astronomy.