It is well known the the majority of the universe is not composed of baryonic matter, the kind of stuff that we and everything we experience is made of composes a mere 4.6% of the total, with dark energy and dark matter making up the next 72 and 23 percent respectively. So the 'bulk' of the universe is pretty much invisible to us, plaguing cosmologists with the responsibility of finding the damn stuff.
One real problem lay in the fact that up till now, a fair amount of the baryonic matter we know should be there was in hiding. This stuff should be easier to find, but it was still missing!
Its all comes down to light, matter needs to either emit, reflect or even absorb electromagnetic radiation in one form or another in order to be directly observed, stuff that normal matter will always tend to do. Thus, with sensitive detectors and the right places to look, we should be able to locate the missing amount.
More work remains to be done in order to map the newly discovered matter, but this is the first step towards greater understanding and further mapping of cosmological distributions.
Full story on the ESA site