Astronomers have known that the universe is expanding with time ever since Edwin Hubble made the discovery in 1929. This started a quest to determine the exact rate of this universal expansion, also known as Hubble's constant which is still in progress.
The base requirement is to find a way to make a reliable measurement of the distance.
There are many ways to make the measurement. One approach that is especially useful is to employ a certain type of supernova called SN 1a as a tool for calibrating distances. This is valuable as the SN 1a is a standard candle. By a standard candle we mean that the object in question has a certain quantity about it that is known and measureable, which in this case is its brightness.
Using the SN 1a as a tool, the astronomer and Nobel Laureate Professor Adam Riess and collaborators announced a new best-value for the measurement Hubble's constant. They made the press release last month at the 231st meeting of the American Astronomical Society in Maryland. The curious part is that their best-value is different from the value measured by a different highly-accurate approach.
In this other approach, the value for Hubble's constant is derived from the Cosmic Microwave Background (CMB). By studying the afterglow of the Big Bang, or the CMB, several quantities can be measured, such as the matter content of the universe and Hubble's constant. Data from the European Planck satellite offer some of the best data with which to study the CMB. Using these data, a lower value of Hubble's constant is measured. The million dollar question is: why would two groups measuring the same quantity get different answers?
It is possible that there are some errors as yet unaccounted for between the team's measurements which can explain the 9 percent difference. At the same time, it is also possible that this difference is real. If so, then these two values can be reconciled by changing the matter content of the universe to include a new particle, such as a "sterile neutrino." This would constitute evidence of one elusive dark matter particle.
At the moment, we do know that these numbers differ by 3.4 sigma, which is pretty good, but not yet enough to start investing in dark matter futures.