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Two million years ago, a massive eruption in the Milky Way caused gases to bubble out into the universe at two million miles per hour (3.2km/h).

Now, astronomers are beginning to see the aftermath of the explosion: billowing clouds of gas towering 30,000 light-years above and below the plane of our galaxy.

Dubbed 'Fermi Bubbles', these mysterious structures shouldn't exist according to current astronomical theory – and scientists are still unable what caused their outburst.

Hubble probed the light from a distant quasar to study the so-called Fermi Bubbles,  The quasar's light passed through one of the bubbles. Imprinted on that light was information about the bubble's speed, composition, and mass. The outflow was produced by a violent event that happened 2 million years ago in our galaxy's core

The enormous structure was discovered five years ago as a gamma-ray glow on the sky in the direction of the galactic centre.

Using the Hubble telescope, Nasa is attempting to find the mass of the material being blown out of our galaxy, which could help determine the cause of the outburst.

'The outflowing clouds we're seeing are only 25,000 light-years away in our galaxy. We have a front-row seat,' said Andrew Fox of the Space Telescope Science Institute in Baltimore.

The giant lobes of Fermi Bubbles glow in nearly uniform gamma rays and appear like two 30,000-light-year-tall incandescent bulbs screwed into the centre of the galaxy 

'We can study the details of these structures. We can look at how big the bubbles are and can measure how much of the sky they are covering.'

The giant lobes of Fermi Bubbles glow in nearly uniform gamma rays and appear like two 30,000-light-year-tall incandescent bulbs screwed into the centre of the galaxy.

The detection of their high-energy gamma rays suggested that a violent event in the galaxy's core violently launched energised gas into space.

To provide more information about the outflows, Professor Fox looked at the ultraviolet light from a distant quasar - a galaxy with a bright active nucleus - that lies behind the northern bubble.

Imprinted on that light as it travels through the lobe is unique information about the velocity, composition, and temperature of the expanding gas inside the bubble.

The study found silicon, carbon, and aluminium, indicating that the gas is enriched in the heavy elements produced inside stars and represents the ancient remnants of star formation.

It measured the temperature of the gas at approximately 9,700°C (17,500°C), which is much cooler than expected.

'We are seeing cooler gas, perhaps interstellar gas in our galaxy's disk, being swept up into that hot outflow,' Professor Fox explained.

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

There are a number of theories attempting to explain why the Milky Way is blowing these enormous bubbles.

Some scientists believe they could have been created by huge jets of accelerated matter blasting out from the supermassive black hole at the centre of our galaxy.

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

Because the bubbles are young compared to the age of the galaxy, and believed to be a short-lived phenomenon, they may be evidence for a repeating event in the Milky Way's history.

Whatever the trigger is, it likely occurs episodically, perhaps only when the black hole gobbles up a concentration of material.

'It looks like the outflows are a hiccup,' Professor Fox said. 'There may have been repeated ejections of material that have blown up, and we're catching the latest one.'

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