The reason the term visible universe is used here is because even though the matter they think they found is hidden it is still matter.  Black holes and other mysterious formations may or may not completely be part of the visible universe.

“Until now, the dispersion measure is all we had,” he said. “By also having a distance we can now measure how dense the material is between the point of origin and Earth, and compare that with the current model of the distribution of matter in the Universe. Essentially this lets us weigh the Universe, or at least the normal matter it contains.”

Dr. Evan Keane of the SKA Organization, and lead author on the paper, was similarly enthused about the team’s discovery. “[W]e have found the missing matter,” he said. “It’s the first time a fast radio burst has been used to conduct a cosmological measurement.”

That is an amazing discovery and in just the last decade or so the astronomical discoveries have been frequent and surprising at the same time.

thanks to universetoday.com for the great info