So does this mean that the amount of theorized dark matter is smaller? I thought dark matter was basically theorized because there are all kinds of observable effects in the universe that indicate there should be more matter than we observe.

Here is the preprint [1].

From a quick glance, the authors look only at a small (relative to the overall size of the universe) region of space and study the emission from the gas between galaxies (specifically, the WHIM [2]). It would seem challenging to draw major conclusions based on this study - and the authors themselves conclude higher quality data of the type they are using is needed.

Some context:

This missing gas or "missing baryons" problem is a very old one [3]. See here [4] for a nice review (Section 4 mentions the missing baryons problem, which really is localized to the cosmologically "recent" universe - we know where the gas was when the universe was older.) It increasingly seems like the missing baryons problem can be resolved by gas living in the WHIM between galaxies. This recent paper [5] argues using Fast Radio Burst data that, in principle, this problem can be solved this way.

[1] https://arxiv.org/pdf/2506.14917

[2] The charmingly named "Warm-hot intergalactic medium" - https://arxiv.org/pdf/astro-ph/0007217

[3] "Baryons" is cosmology jargon for ordinary matter - gas and plasma.

[4] https://arxiv.org/pdf/1512.00086

[5] https://arxiv.org/abs/2409.16952