We learn that there are four fundamental forces in nature: gravity, electromagnetism, and the strong and weak nuclear forces. One research group led by Dr. Lijing Shao from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has recently set out to test the possibility of a fifth force of nature.
We already know that gravity acts on matter. For example, gravity reliably pulls us to the Earth, keeps the Earth orbiting the Sun, and causes the Sun to orbit the center of the Milky Way.
The proposed new force would act preferentially on the dark matter. Dark matter is a component of the universe which dominates the mass of the universe, meaning that there is more dark matter than the protons, neutrons and electrons that we are made of, yet we cannot see it. This "fifth" force would pull ordinary matter towards the dark matter, or the other way around.
To search for evidence of a fifth force, Dr. Shao and his team studied a binary star system consisting of a pulsar and a white dwarf in orbit about each other. These two "degenerate" stars are so densely packed that their gravity is very high too, making them excellent candidates to study the forces pulling on them.
This experiment must be done by observing these objects in space, as degenerate material cannot be placed in a terrestrial lab. For example, a single teaspoon of a pulsar would fall through the Earth!
If a fifth force exists, then there would be a different pull on the denser pulsar relative to the "lighter" white dwarf. This should result in a detectable change in their orbits which was not seen. While this experiment did not show the anticipated change, this will not stop us from reaching out to understand the nature of dark matter.