Searching for dark matter through secondary signals like gamma rays or magnetic field interactions.Dark matter indirect detection is a strategy for finding dark matter not by catching the particles themselves, but by looking for the secondary signals they produce when they interact with ordinary matter or with each other. Since dark matter is invisible and passes through everything, direct detection experiments (which wait for a dark matter particle to bump into an atomic nucleus) are extremely difficult. Indirect detection instead searches for the byproducts of dark matter annihilation or decay — such as gamma rays, neutrinos, or even the heating of dense regions — which could be observed by telescopes and other instruments. The idea is that if dark matter particles occasionally collide and annihilate, they would produce a distinctive spectrum of radiation that we could spot coming from regions where dark matter is expected to be dense, like the centers of galaxies or the Sun.
The concept of indirect detection emerged in the 1980s, with early work by theorists like Lawrence Krauss and Frank Wilczek, who suggested that dark matter annihilation in the Sun could produce detectable neutrinos. The field gained momentum in the 2000s with the launch of gamma-ray telescopes like Fermi-LAT.
The mechanism relies on the fact that dark matter, despite being 'dark', is not completely inert. In many theoretical models, dark matter particles can annihilate with each other, producing standard-model particles like quarks, leptons, or photons. These products then decay or radiate, creating gamma rays or other cosmic messengers. The Earth itself can act as a detector: if dark matter particles pass through it, they might scatter and become trapped in the Earth's core, where they accumulate and annihilate, producing a faint neutrino signal. Similarly, the Sun could capture dark matter and emit neutrinos from its core. Another approach looks for the gravitational or thermal effects of dark matter on its surroundings, such as the way it might heat up dense regions or leave an imprint on the cosmic microwave background.
Indirect detection is used in astrophysical observations, such as searching for gamma-ray excesses from the galactic center or dwarf galaxies, and in neutrino telescopes like IceCube, which look for neutrinos from dark matter annihilation in the Sun or Earth.
The Earth's shadow can act as a natural filter for dark matter, as the planet blocks a portion of the incoming dark matter flux, creating a detectable asymmetry in the signal.
| Om | variable | Om diagnostic value — om diagnostic |
| z | variable | Redshift — redshift |
| H | variable | Hubble parameter at redshift z — hubble parameter kilometre per second per megaparsec |
| H_0 | variable | Hubble constant at present epoch — hubble constant kilometre per second per megaparsec |
