By Dr David Whitehouse BBC News Online science editor

The Universe will expand for ever, at an ever-increasing rate, Nasa scientists have announced.

They base their conclusion on new data obtained by the Microwave Anisotropy Probe (Map) satellite, which has been orbiting the Sun beyond the Moon since shortly after its launch in 2001.

Map data also confirm previous findings that most of the Cosmos is composed of mysterious "dark energy" that is causing the expansion of the Universe to accelerate.

Atoms - the basic components of matter that can give off light - comprise only a few per cent of the Universe. As one astronomer put it: "To the Universe, stars and planets are minor impurities."

Map was launched in 2001 to make its way to the L2 Lagrange point of gravitational balance between the Earth, Moon and Sun.

Big Bang echo

It is the first probe to be positioned at L2, which is four times further away than the Moon, and which follows the Earth and the Moon around the Sun.

Map's focus is the Cosmic Microwave Background (CMB) radiation. The CMB was first detected in 1965 by Arno Penzias and Robert Wilson of the Bell Telephone labs in the US.

Map satellite was launched in 2001

It has been called the "echo" of the Big Bang - the event that created the Universe about 15 billion years ago.

The CMB is radiation that formed about 400,000 years after the Big Bang, when the Universe had cooled to such a degree that hydrogen atoms could exist.

In 1992 the Cosmic Background Explorer (Cobe) satellite detected fluctuations in the CMB that were attributed to the first structures to form in the Universe - the so-called seeds of galaxies appearing in the vast clouds of hot gas that was all the Universe consisted of at the time.

Ever-cooling

Astronomers believe that the CMB contains a great deal of information about the origin and fate of the Universe.

Measurements of the CMB will allow cosmologists to determine basic parameters of the Universe, for instance whether it will expand for ever, or collapse, or whether its expansion will accelerate or slow down.

Able to scan the whole sky every six months, the Nasa satellite is producing maps of the CMB with unprecedented accuracy.

Map's first release of data confirms previous results obtained by the Boomerang balloon-based detector that flew over Antarctica in 2000.

It shows that "dark energy" dominates the Universe, causing the expansion of the Cosmos to accelerate.

This means that eventually all matter in the Universe will be scattered ever more thinly and, as the stars go out and the galaxies fade, all will become an ever-cooling thin gas.