What is the universe made of? This is one of the burning questions confronting today’s astrophysicists and cosmologists.

The night sky is full of stars. With even a small backyard telescope, more stars and distant galaxies, islands of stars, become visible.

The “standard model” of quantum theory categorizes the elementary particles much as the periodic table does with the chemical elements. In the standard model we find six quarks that make up protons, neutrons and mesons, six leptons that include electrons and neutrinos, and five bosons that are force carriers such as photons, gluons and the Higgs boson.

These particles constitute all we know of the visible universe. Everything we see, including galaxies, stars, planets and us, is made from these particles.

Yet astronomical observations suggest that there is six times more matter than we can see. We can detect invisible matter by the effect that it has on the matter we can see.

That story begins in the 1930s when astronomer Fritz Zwicky noticed that galaxies in clusters were moving very fast, too fast to stay gravitationally bound. Zwicky estimated that it required 100 times more matter than he could see to hold the galaxies together by gravity. He coined the term “dark matter” for this missing mass.

Nobody believed him.

Then, in the 1970s, astronomer Vera Rubin was measuring the speed of rotation of various galaxies when she noticed that stars in the outer parts of the galaxies were moving faster that they should be.

According to Newton’s law of gravity, the gravitational force is proportional to the inverse square of distance. That means that stars farther from the center should be moving slower than those nearer to the center as the galaxy rotates. This is similar to our solar system where planets farther out from the sun move slower in accordance with the inverse square law.

Rubin’s measurements showed that all stars in a galaxy rotate at the same speed, even those on the outer fringes where there are few stars to provide the necessary mass. This is impossible unless the law of gravity is wrong or there is more mass in the galaxy than we can see.

The law of gravity has been widely tested, including Einstein’s general theory of relativity, which was proved correct when it correctly described the precession of Mercury’s orbit and bending of light around the sun during the total eclipse in 1919.

Rather than abandon the 300-year-old laws of gravity, the solution to the problem was to postulate that an unseen kind of matter formed a halo around the galaxies instead of being clumped up like ordinary matter to provide the mass necessary to account for the stars’ motions.

Only about 4% of all visible matter is in the form of stars. Another 11% is in the form of interstellar gas. The remaining 85% is dark matter.

It is “dark” because we cannot see it but also because we have no idea what it is. For reasons too complicated to note here, black holes have been ruled out as dark matter candidates to remain consistent with the standard model, which deals with the rate at which new elements were formed in the early universe.

Astrophysicists have proposed different theories to account for dark matter, including supersymmetry and string theory. Both of these theories resolve the dark-matter problem, but neither has any experimental verification.

Other theoretical particles known as WIMPs, or weakly interacting massive particles, and axions have been proposed, but so far no evidence has been found of either despite experiments in Germany, at CERN and elsewhere.

So, the problem of the missing matter remains unsolved. To make matters worse, in the 1980s astronomers noticed that the rate at which the universe is expanding is accelerating, leading to the concept of dark energy, which is a topic for another time.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.