[nz]

It depends a lot on what counts as "observation" or "measurement". For example, nobody with a telescope (of any magnification and resolution) would postulate the existence of dark matter. But, one person (Zwicky, 1933), _did_ postulate the existence of dark matter, precisely because he combined the what he saw through the telescope (light), with his understanding of physics (gravity and mass), and was able to infer that there must be matter that _cannot_ be seen with a telescope.

It's always possible (is in fact likely), that we have many (metaphorical) people who peer through (metaphorical) telescopes, but lack a (metaphorical) theory of gravity, preventing them from even considering the existence of (metaphorical) dark matter.