What is the best rationale for the existence of dark matter according to scientists?

The best rationale for the existence of dark matter is that it serves as a working hypothesis to explain real observations in the universe. Scientists have observed various phenomena, such as the rotation curves of galaxies and the gravitational lensing of light, which cannot be fully explained by the presence of visible matter alone. The velocity at which galaxies rotate suggests that there is significantly more mass present than what can be detected through electromagnetic radiation. This discrepancy led to the hypothesis of dark matter as a means to account for the additional gravitational effects observed.

By positing the existence of dark matter, scientists can construct models that match these astronomical observations more accurately, thus providing a cohesive understanding of the structure and behavior of the universe. It is a necessary component of our current understanding of cosmology and gravitational dynamics, making it a critical element in explaining why galaxies behave as they do in a way that is consistent with observational data.