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The Milky Way is blowing huge, mysterious bubbles that stretch for tens of thousands of light years.

These 'Fermi' bubbles, which are mostly made up of gamma rays, were discovered four years ago by Harvard physicist Douglas Finkbeiner - and scientists have been trying to explain them ever since.

Now a group of US researchers has used data from the Fermi Gamma-ray Telescope to create a 'portrait' two bubbles stretching out above and below our galaxy.

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The Milky Way is blowing huge, mysterious 'Fermi' bubbles that stretch for tens of thousands of light years

Dmitry Malyshev, at the Kavli Institute for Particle Astrophysics and Cosmology in Stanford, found that the bubbles have very clear outlines and are fixed at each pole of the Milky Way.

The bubbles themselves, he claims, glow in nearly uniform gamma rays and appear like two 30,000-light-year-tall incandescent bulbs screwed into the centre of the galaxy.

But according to current astrophysical theories, these gamma rays shouldn't be there, and scientists have been unable to find a source.

There are a number of theories. For example, they could have been created by huge jets of accelerated matter blasting out from the supermassive black hole at the centre of our galaxy.

The bubbles extend 30,000 light-years across. Hints of the bubbles' edges were first seen in X-rays (blue). The gamma rays mapped by Fermi (shown in magenta) extend much farther from the galaxy's plane

Five Years On: Fermi's quest for knowledge since 2008

Or they could have been formed by a population of giant stars, born from the plentiful gas surrounding the black hole, all exploding as supernovae at roughly the same time.

Another theory is they are the result of collisions between dark matter particles that result in their annihilation, emitting charged particles in the process.

'There are several models that explain them, but none of the models is perfect,' said Dmitry Malyshev, a postdoctoral researcher at the Kavli Institute. 'The bubbles are rather mysterious.'

From the vantage point of most Earth-bound telescopes, all but the highest-energy gamma rays are completely screened out by our atmosphere.

It wasn't until the era of orbiting gamma-ray observatories like Fermi that scientists discovered how common extra-terrestrial gamma rays really are.

Pulsars, supermassive black holes in other galaxies and supernovae are all gamma rays point sources, like distant stars are point sources of visible light, and all those gamma rays had to be scrubbed from the Fermi data.

Hardest to remove were the galactic diffuse emissions, a gamma ray fog that fills the galaxy from cosmic rays interacting with interstellar particles.

'Subtracting all those contributions didn't subtract the bubbles,' said KIPAC postdoctoral researcher Anna Franckowiak. 'The bubbles do exist and their properties are robust.'

In other words, the bubbles don't disappear when other gamma-ray sources are pulled out of the Fermi data – in fact, they stand out quite clearly.

The researchers plan to continue to gather data on the bubbles, while attempting to explain how they got there.

A giant gamma-ray structure was discovered by processing Fermi all-sky data at energies from 1 to 10 billion electron volts, shown here. The dumbbell-shaped feature (centre) emerges from the galactic core and extends 50 degrees north and south from the plane of the Milky Way, in the sky from the constellation Virgo