A recent article from a popular astronomy website tells of discovery of missing mass (not dark matter) that has puzzled astronomers for some time. Apparently, the discovery involves enhanced electron density in filaments associated with superclusters of galaxies. How were astronomers able to determine that this baryonic mass was missing in the first place, and what percentage of total baryonic mass did it entail?
The article answers the question, though it does not lean on it:
Nor is this a surprise, orbital velocities and lensing are really the only reliable tools for weighing things at a very long distance.
Another thing to note is that the dark matter (what ever it may be) interacts gravitationally, too. So just seeing evidence of more mass than you can account for does not distinguish between some unexpected ordinary matter and dark matter, which is why the x-ray band observation is so interesting.