Like the chocolate bar that bears its name, the Milky Way galaxy is lighter than it might first appear.

A long standing advertising slogan for the chocolate Milky Way was “the sweet you can eat between meals”, based on it being a light-weight snack.

Now scientists have discovered that the real Milky Way, the galaxy containing the sun and its family of planets, is lighter than was previously thought.

In fact, it has about half the mass of one of its closest neighbours, the Andromeda galaxy, 2.5 million light years away.

Both are similar types of spiral galaxy and the largest in a clump of galaxies known as the Local Group.

Scientists believe Andromeda’s extra weight must be in the form of dark matter, an invisible substance thought to bind galaxies together with its gravity.

The galaxy, which is about the same size as the Milky Way, is thought to contain twice as much dark matter as our galactic home.

Previous studies were only able to measure the mass enclosed within the galaxies’ inner regions. The new study included the mass of outer region invisible matter.

Data on the known distances between galaxies in the Local Group were used to calculate the total masses of Andromeda and the Milky Way.

The findings, from a team led by Jorge Penarrubia at the University of Edinburgh, appear in the journal Monthly Notices of the Royal Astronomical Society.

Penarrubia said: “We always suspected that Andromeda is more massive than the Milky Way, but weighting both galaxies simultaneously proved to be extremely challenging.

“Our study combined recent measurements of the relative motion between our galaxy and Andromeda with the largest catalogue of nearby galaxies ever compiled to make this possible.”