Pulsars or dark matter as the source of excess cosmic positrons
In plain words
There are more positrons, antimatter electrons, at high energies than collisions of ordinary cosmic rays with gas can make. Nearby pulsars or dark matter could supply them.
Precise statement
The positron fraction e+/(e+ + e-) rises above about 10 GeV (PAMELA, AMS-02), and the AMS-02 positron flux shows a high-energy drop-off near several hundred GeV. Identify the dominant extra source (nearby pulsars and their wind nebulae, dark-matter annihilation or decay, or secondary production in sources) by fitting the spectrum, its cutoff and the upper limits on dipole anisotropy.
What would settle it
Detection or exclusion of the anisotropy or spectral structure predicted for nearby pulsars, combined with TeV electron spectra and diffusion constraints near pulsars.
Status in the literature
TeV halos around Geminga and Monogem seen by HAWC show that pulsars release electron-positron pairs, but slow diffusion near them limits their contribution at Earth and the share remains debated.