Dark matter is believed to be responsible for more than 70 percent of the total mass of the universe, however somehow we can't find any, even while regular matter represents only 4 percent of the total mass. This means only two things: either dark matter presents weak interactions towards ordinary matter or dark matter doesn't exist at all. Right now researchers from the St Andrews University are more inclined to take into consideration the second supposition.

They believe that the motion of dwarf galaxies orbiting around the Milky Way is more likely to be explained by the Modified Newtonian Dynamics rather than through dark matter.

"MOND was first suggested to account for things that we see in the distant universe. This is the first detailed study in which we've been able to test out the theory on something close to home. The MOND calculations and the observations appear to agree amazingly well. We've also found some exciting tidal effects predicted by MOND that we should be able to test through future observations and simulations," said Garry Angus from the St. Andrews University.

Basically, except the speed of light, Planck's constant and some other universe constants, MOND adds a new constant dubbed (a0). When this acceleration is exceeded objects tend to follow precisely the Newton's second law of motion, however if acceleration drops below this constant, gravity begins to decay with distance from a mass, rather than the square distance.

For example, an object falling under the force of Earth's gravity experiences an acceleration 100 billion times bigger than a0, thus the a0 constant has no influence on the motion of the object. Albeit, while objects are accelerated slowly, such as the galaxies and star clusters, the a0 acceleration begins to exert a powerful influence on the resultant gravitational force.

The effects of MOND can be clearly observed on dwarf galaxies experiencing the tidal forces exerted by our galaxy, effect which is basically inexistent in classical Newtonian Mechanics, but which makes a big difference on the large scale.

"In these dwarf galaxies, the internal gravity is very weak compared to the gravity of the Milky Way. MOND suggests that the Milky Way is a bit like a bank that loans out gravity to nearby dwarf galaxies to make them more stable. However, there are conditions on the loan: if the dwarf galaxies start to approach the bank, the loan is gradually reduced or even cancelled and the dwarfs must pay it back. In two galaxies, we've seen what could be signs that they've come too close too quickly and are unable to repay the loan fast enough. This appears to have caused disruption to their equilibrium," said Angus.

By calculating the mass to light ratio in all eight satellite dwarf galaxies and the random velocities of stars inside them, Angus revealed that six of the satellite galaxies accurately followed the predictions made with MOND, while two other galaxies suggested tidal effects. Alternatively, the light coming from the two was to dim to make accurate measurement, meaning that they could still fall very well into the predictions made with MOND.

"These tidal effects can be tested by updating the 13-year-old luminosity of Sextans and making accurate observations of the orbits of Draco and Sextans around the Milky Way. We also need to carry out some detailed simulations to understand the exact mechanisms of the tidal heating. Even without direct detection, the dark matter theory is difficult to prove or refute and although we may not be able to prove whether MOND is correct, by carrying out these kind of tests we can see if it continues to hold up or if it is definitely ruled out," said Angus.