The behaviour of the Bullet cluster – the poster-child for the existence of dark matter – is provoking some cosmologists to propose that there might be a fifth fundamental force.
Most physicists are confident that they have uncovered all the forces that affect ordinary matter: gravity, electromagnetism, and the strong and weak nuclear forces. If there was a fifth force that influenced only dark matter, however, it could easily be at work without us realising, says Glennys Farrar of New York University.
She and her student Rachel Rosen believe they have found evidence for just such a force in the so-called Bullet…
