Only 4.9% of the matter in the Universe is visible. The rest is known as dark matter (26.8%) and dark energy (68.3%). Finding out what dark matter consists of is a major challenge for modern science.
Particles decay into other particles over time. Decay channels are the possible transformations a particle can undergo as it decays. For example, the Higgs could decay into several different channels, such as two photons or two W bosons or two Z bosons.