That's exactly what it is. It's the difference between estimating mass based on visible light and estimating mass based on gravitational effects. If both thos

That's exactly what it is. It's the difference between estimating mass based on visible light and estimating mass based on gravitational effects. If both those estimates are accurate, it implies the existence of a lot of non-luminescent matter. So some scientists look for "dark matter", while others try to find flaws with either estimation.