The statement that every elementary particle has mass is not entirely accurate. While many elementary particles, such as quarks and electrons, have mass, there are some elementary particles that are believed to be massless, such as photons (particles of light) and gluons (particles that mediate the strong nuclear force).
Regarding dark matter, it is important to clarify that dark matter is not composed of elementary particles. Dark matter is a hypothetical form of matter that does not interact with light or other forms of electromagnetic radiation, making it invisible to traditional detection methods. Its existence is inferred through its gravitational effects on visible matter and the large-scale structure of the universe.
The evidence for the existence of dark matter comes from various astronomical observations and cosmological studies. For instance, measurements of the rotation curves of galaxies, the motions of galaxy clusters, and the pattern of fluctuations in the cosmic microwave background radiation all point towards the presence of an additional, unseen mass that cannot be accounted for by known visible matter. These observations suggest that there must be a significant amount of matter in the universe that interacts gravitationally but is otherwise difficult to detect.
Scientists have proposed various hypothetical particles as potential candidates for dark matter, such as weakly interacting massive particles (WIMPs), axions, and sterile neutrinos. These particles, if they exist, would be different from the elementary particles we are familiar with in the Standard Model of particle physics. Experimental efforts are underway to directly detect dark matter particles or indirectly observe their interactions through their possible effects on other particles.
In summary, while some elementary particles do have mass, not all do, and dark matter is not composed of elementary particles. The evidence for dark matter primarily arises from its gravitational effects on visible matter, and its nature and composition remain an active area of research and exploration in cosmology and particle physics.