Mysterious lumps in gas clouds connecting stars in the Milky Way could comprise a significant amount of the galaxy's missing matter, according to Australian scientists.

Key points: Noodle-shaped lumps lie in gas between stars in the Milky Way

Noodle-shaped lumps lie in gas between stars in the Milky Way Discovery challenges understanding of gas in Milky Way

Discovery challenges understanding of gas in Milky Way Lumps in gas work like lenses, focusing and defocusing radio waves

Lumps in gas work like lenses, focusing and defocusing radio waves They may make up a substantial amount of galaxy's dark matter

The invisible structures, reported in the journal Science, are thought to be shaped like noodles, lasagne sheets or possibly hazelnuts.

"These observations provide us with a new understanding of the galaxy's interstellar gas which is important for recycling stellar material for new star formation," said the study's lead author Dr Keith Bannister of the CSIRO.

The structures appear to be lumps in the thin electrically charged gas that lies between the stars in our galaxy.

"These lumps of gas work like lenses, focusing and defocusing the radio waves from more distant objects, making them appear to strengthen and weaken over a period of days, weeks or months," Dr Bannister said.

"It's the same as looking at a light through a wine glass, as you move the wine glass you'll see the shape of the light bulb changing and getting brighter and fainter."

Astronomers first detected the mysterious clouds 30 years ago when they saw radio waves from a bright quasar in a distant galaxy varying wildly in strength.

However, back then scientists had trouble working out what they were because they only had two frequencies to look at.

Powerful telescope enabled detailed study of gas

Using the powerful Australia Compact Array Telescope, Dr Bannister and colleagues were able to study the gas cloud in more detail as it passed in front of a quasar called PKS 1939-315 in the constellation Sagittarius.

"In our new data instead of just two channels, we have 9,000 channels," Dr Bannister said.

"We saw two bumps in the data — it first got brighter for a few days, it then became much fainter before getting brighter again and finally going back to normal."

The optical light from the quasar did not change while the radio lensing took place, indicating that optical surveys for these events would not detect them.

The authors estimate the lens is about the size of the Earth's orbit around the Sun and lies approximately 3,000 light-years away.

The new observations have allowed Dr Bannister and colleagues to develop their first estimates of the clouds' shape.

"This means we're able to start ruling out different geometries for these objects — such as solid balls of gas or bent sheets of gas. It's like going from black and white to colour," Dr Bannister said.

There are still a couple of geometries that Dr Bannister and colleagues are looking at, including sheets of gas seen edge-on or hollow noodle-like cylinders.

"Another possibility involves lumps of gas shaped like a hazelnut and comprising a spherical shell of plasma surrounding a core of cooler neutral gas, with gravity providing the confinement to keep the cloud together," he said.

"These things are all over the Milky Way and comprise a fair proportion of the galaxy's overall mass."

Lenses could help explain missing mass problem

Dr Bannister said these lenses could help explain what astronomers call the missing mass problem.

"Current models of nucleosynthesis in the first few minutes after the Big Bang 13.8 billion years ago predict that far more hydrogen, helium and lithium was produced than what we can actually find," he said.

"The leading candidate is the hot plasma contained in the filaments and knots of the web-like, large-scale structure of the universe. Our lenses provide another candidate."

The authors still do not know how these invisible lenses form or their composition and temperature.

"This is important because we don't know much about these things or what they do to the structure of the galaxy and the gas in the Milky Way," Dr Bannister said.