Scientists have revealed that since the Big Bang, the universe has lost up to 5 percent of dark matter.
Dark Matter, Ordinary Matter, And Dark Energy
Dark matter composes about a quarter of the universe while ordinary matter only makes up just 5 percent. The rest of the universe is composed of dark energy, which is believed to drive the expansion of the universe.
Scientists discovered dark matter in the 1930s after noticing that galaxies move as if they were under the influence of gravity. Galaxies also rotate at speeds that in the absence of an invisible force would tear them apart.
Cosmic Microwave Background
Dark matter is invisible because it does not reflect, absorb or emit light. One of the means that scientists analyze dark matter is by studying cosmic microwave background (CMB), the so-called "echo of the Big Bang." CMB is the thermal radiation left over from the Big Bang. Researchers use it as an astronomical time capsule to understand the early universe.
Using the Planck telescope, which looks at fluctuations of CMB, researchers found huge variations in the cosmological parameters from today's observable universe and with what is seen during the recombination era about 300,000 years after the Big Bang.
Researchers said that the discrepancy can be best explained by a decrease in the proportion of dark matter.
CMB, DDM Model, And Lambda-Cold Dark Matter Model
By comparing CMB data from Planck with the decaying dark matter (DDM) model that assumes the amount of dark matter could change, and the standard ΛCDM (Lambda-cold dark matter) model that assumes dark matter is stable, researchers found that DDM is better at producing something like the universe that we have today. The DDM hypothesis proposes that there was more dark matter in the early universe but part of it has decayed.
Two Kinds Of Dark Matter
The researchers said that the universe likely had two kinds of dark matter, one that decays into other particles and another that remains stable over a long period of time.
Calculations made by the researchers also suggest that the universe has lost between 2 and 5 percent of dark matter since the Big Bang, which can be attributed to dark matter particles decaying over time.
"This means that in today's universe there is 5 percent less dark matter than in the recombination era," said Igor Tkachev from the Institute for Nuclear Research of the Russian Academy of Sciences.