The statement that every elementary particle has mass is not entirely accurate. While many elementary particles do have mass, there are also elementary particles that are massless, such as photons (particles of light) and gluons (particles that mediate the strong nuclear force).
As for dark matter, it is true that the evidence for its existence primarily comes from its gravitational effects rather than direct observation. Dark matter is a hypothetical form of matter that does not interact with light or other forms of electromagnetic radiation, making it difficult to detect using traditional observational techniques. However, its presence can be inferred from its gravitational influence on visible matter and the large-scale structure of the universe.
The evidence for dark matter comes from various sources, including observations of galactic rotation curves, gravitational lensing, and the cosmic microwave background radiation. These observations suggest that there is additional mass in the universe that cannot be accounted for by visible matter (such as stars and galaxies) or the known elementary particles. The term "dark matter" is used to describe this unidentified form of matter that seems to contribute significantly to the total mass of the universe.
It's important to note that the nature of dark matter is still an active area of research, and scientists are exploring various possibilities for its composition. Numerous experiments are underway to directly detect dark matter particles or to study their properties indirectly through their interactions with normal matter. While the precise nature of dark matter remains unknown, its existence is strongly supported by the observational evidence.