By Helen Briggs
Science reporter, BBC News
A Nasa space probe measuring the oldest light in the Universe has found that cosmic neutrinos made up 10% of matter shortly after the Big Bang.
Temperature fluctuations in the afterglow from the infant Universe: red regions are warmer and blue are cooler
Five years of study data also shows that the first stars took over half a billion years to light up the Universe.
WMAP launched in 2001 on a mission to measure remnants of light left over from the Big Bang.
Scientists say it is collecting a "treasure trove" of information about the Universe's age, make-up and fate.
The Wilkinson Microwave Anisotropy probe (WMAP) is mapping the Cosmic Microwave Background (CMB) radiation in the sky. This is the oldest light in the Universe, shifted to microwave wavelengths as the Universe expanded over 13.7 billion years.
Scientists say the Universe is bathed in this afterglow light, and its patterns across the sky contain details about the history, shape, content, and ultimate fate of the Universe.
WMAP provides new evidence that a sea of cosmic neutrinos flows throughout the Universe.
Dr Joanna Dunkley of the University of Oxford, UK, and Princeton University, US, is a member of the WMAP team.
She told the BBC News website: "We see patterns in light, light that has been travelling for billions of years, affected in the early infancy of the Universe by whatever the Universe was composed of at that point.
"We expected to see neutrinos. It's a nice piece of evidence that they are in the Universe at large and affecting the light signals we see."
Neutrinos made up a much larger part of the early Universe than they do now. Cosmologists believe neutrinos were present in such abundance shortly after the Big Bang that they influenced the initial development of the Universe.
Thus, the light that WMAP now sees as microwaves gives an impression of conditions billions of years ago.
Scientists say the WMAP data confirms theories based on the amount of helium observed today. The hot, dense young universe was a nuclear reactor that produced helium and, according to particle physicists, a sea of neutrinos must have been present.
WMAP also provides evidence that the first stars took a very long time to glow - producing a cosmic "fog" caused by electrons scattering microwaves.
Dr Dunkley said the WMAP data gave a picture of when the first stars began to burn in the Universe.
She said: "We basically have the first evidence that how the first stars switched on was a long, drawn-out process that took half a billion years. We weren't able to see that before."
WMAP is mapping microwave light from its vantage orbit about four times further from the Earth than the Moon, following our planet around the Sun.
Full details of the research have been submitted for publication in the Astrophysical Journal.