Positrons are the antiparticles of electrons, and as such, they cannot coexist with normal matter of which our world is made of. When a positron encounters an electron both particles annihilate under the emission of a characteristic signature: the 511 keV electron-positron annihilation emission. This emission has been detected for the first time by gamma-ray telescopes in the seventies towards the centre of the Milky Way, but until today, the source of the positrons is still unknown.

For the first time, the SPI telescope aboard ESA's INTEGRAL gamma-ray observatory has now provided a map of the entire sky in the 511 keV electron-positron annihilation emission. This maps shows a strong signal coming from the direction of the galactic centre, while the sky is surprisingly dark in all other directions. No object is known in our Galaxy that could easily produce such a particular emission morphology.

Thermonuclear explosions of white dwarfs, also called Type Ia supernovae, could in principle create large amounts of positrons by the radioactive decay of stellar debris. But these objects are not expected to be concentrated in the galactic centre region. Novae, which consist of thermonuclear runaways on the surface of white dwarfs, are believed to be abundant near the centre of the Milky Way, but it is not clear how they could produce the large amounts of positrons required to explain the observed 511 keV emission intensity. Close binary systems composed of solar type stars and a massive compact object, such as a neutron star or a black hole, tend also to be more abundant towards the centre of the Galaxy, but a substantial fraction of them should also be found in the galactic disk. However, only very dim 511 keV emission is seen from the disk of the Galaxy, much too faint as that expected from close binary systems. In the struggle for finding the source of positrons even the annihilation of exotic new particles, also known as light dark matter, has been proposed. But the observed 511 keV emission distribution seems to be difficult to reconcile with the expected spatial distribution of these particles within the Milky Way.

Definitely, the source of galactic positrons remains for the moment one of the biggest mysteries in astronomy. Luckily, observations with the SPI telescope aboard INTEGRAL will continue during the next years, and the growing amount of observations will allow to provide for the first time a detailed image of the 511 keV emission in our Galaxy. Nearby positron sources could even be detected individually, thanks to the excellent angular resolution and sensitivities of the instruments aboard INTEGRAL. We therefore expect that INTEGRAL will finally resolve the puzzle of the mysterious source of positrons in the Milky Way.