La llamada materia oscura constituye el 85% del contenido de masa del universo. Los investigadores la llaman "oscura" porque no se nota nada en ella, excepto su gravedad. Sin embargo, esto se puede detectar bastante bien. Sin la materia oscura, las galaxias se moverían de forma diferente a como se ha demostrado, y el universo tendría una estructura diferente. Así que los físicos necesitan la materia oscura para explicar el cosmos. Lástima que todavía no sepan de qué está hecho. Hay candidatos para ello: MACHOs (objetos compactos masivos del halo), WIMPs (partículas masivas de interacción débil) e incluso agujeros negros…

For some time now, physicists have been thinking about an elementary particle that has very little mass, no electric charge and no spin (quantum angular momentum). It would interact very little with other particles because of these properties and would therefore be a good candidate for dark matter, which is characterized by just that. But the axion is also used in physics because in the neutron, a neutral nuclear particle, the charge of the quarks of which it is composed is so perfectly distributed that it is not at all apparent to the outside world that there are balancing…

The neutron is, as suggested by its name, electrically neutral. Nevertheless, it still contains electrical charges. More specifically, it is made up of one up quark (charge: 2/3 of an electron charge e) and two down quarks (charge: -1/3 e each). In total, 2/3 + 2*(-1/3) equals exactly 0. But the neutron is not one-dimensional. It has a diameter of at least 1.7 * 10-15 meters, and when three components have to be spread out over any distance, even with an overall zero charge, some difference in charge should still be detectable. Calculations from theory say that a neutron should have an electrical dipole…