Intergalactic Collision Constrains Dark Electromagnetism
Dark matter, by definition, does not participate in electromagnetic interactions. But some theories include a dark matter analogue of this familiar force. These theories imply that dark matter particles might possess positive or negative “dark charges” and thereby experience attractive or repulsive forces. Now Kevin Schoeffler at Ruhr University Bochum in Germany and colleagues have sought evidence of such interactions in the distribution of dark matter around a pair of colliding galaxy clusters [1]. Finding none, the researchers have set an upper bound on the strength of possible dark electromagnetism.
In a previous study, Schoeffler and colleagues simulated the collision of two clouds of plasma that pass through one another [2]. They found that, as the clouds merge, electromagnetic instabilities develop. These instabilities redistribute energy from the clouds’ opposing flows, slowing the clouds while broadening their range of temperatures.
The researchers’ current work was motivated by the recognition that an implementation of their colliding-plasma scenario is unfolding at a cosmic scale several billion light years away. In this natural experiment, one cluster of galaxies—the Bullet Cluster—is moving away from another larger galaxy cluster after passing through it at high speed. The plasma components of each cluster are behaving as expected: They have slowed down because of electromagnetic interactions. But each cluster’s dark matter—the distribution of which astronomers infer through gravitational-lensing observations—appears to be affected by gravity alone. The absence of a measurable dark matter self-interaction means that any dark electromagnetism must be far weaker than its conventional counterpart. This, Schoeffler says, rules out many of the simplest models and could guide future direct-detection experiments.
–Marric Stephens
Marric Stephens is a Corresponding Editor for Physics Magazine based in Bristol, UK.
References
- K. Schoeffler et al., “Can plasma physics establish a significant bound on long-range dark matter interactions?” Phys. Rev. D 111, L071701 (2025).
- N. Shukla et al., “Slowdown of interpenetration of two counterpropagating plasma slabs due to collective effects,” Phys. Rev. E 105, 035204 (2022).