https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...
The average mass density of the universe is known within a few percent.
http://hyperphysics.phy-astr.gsu.edu/hbase/astro/denpar.html
From reading the article, I believe this is literally just a finding about how many galaxies into which the known amount of matter was clumped in the distant past, i.e., more galaxies with less average mass.
We've known for a while now that there seems to be more galaxies out there, heck before the hubble went up we didn't even knew how much there is. There are also indications that there is considerably more intergalactic material out there than we've suspected before.
However the problem is that "dark matter" is also needed on local scales, including our solar system; this is a good read http://scienceblogs.com/startswithabang/2013/01/18/why-the-u.... So "dark matter" is needed to explain our current model of gravity both local and global scales; it is important to note tho that "dark matter" is a placeholder it doesn't necessarily have to be a new type of matter (e.g. AXIONs), some irregular mode for normal matter (e.g. WIMPs) or or it can be regular matter which isn't easily observable (e.g. MACHOs) or it can be also be any combination of the above, and things we haven't even considered yet.
Alternatively we might need to rethink our understanding of gravity, and of the existing forces in general, a good point to note is that gravity isn't a traditional force in the first place which already sets it apart from the bunch (it's a property of the distortion of spacetime, it also is only attractive unless dark energy is somehow a very low/zero value property of the gravitational field).
So gravity might not work like we think it does, it might be hiding another elementary force, or it might be that gravity on certain scales/conditions splits into multiple forces similarly to how the electroweak force works (at some point during/prior to the big bang all forces/fields were unified at least in theory).
Mainstream Physics have been stuck somewhat for a fairly long time on certain aspects, there is also quite a bit of misconception about what works and what doesn't for example most people think that quantum gravity doesn't work which it does, you can use it for everyday life and for many other things, you can't use it during the big bang (and shortly after) and it breaks down near very strong fields e.g. a singularity which is also the internal killswitch or "mathematical contradiction" embedded within general relativity where gravity and spacetime as we know it break down.
"found that 10 times as many galaxies were packed into a given volume of space in the early universe than found today. Most of these galaxies were relatively small and faint, with masses similar to those of the satellite galaxies surrounding the Milky Way. As they merged to form larger galaxies the population density of galaxies in space dwindled. This means that galaxies are not evenly distributed throughout the universe's history,"
I read this as they did not find new visible matter, but they discovered that the visible matter was distributed in many more small galaxies compared with today. I could be wrong though, I'm by no means an expert in this field.