For almost 5 decades now, the idea of dark matter and dark energy constituting about 96% of the universe has gained widespread acceptance among the scientific community. Three centuries ago, the famous mathematician and physicist Isaac Newton introduced his theory of universal gravitation, which explained the relevance of a newly discovered force, the force of gravity. He explained that gravity is a force that expands throughout the universe and is exerted by all objects with the presence of mass. He reasoned that if an object has mass, it will exert a force field that attracts all other objects in the universe. The strength of this force field is totally dependent upon the distance between the objects and the masses of the objects. If the mass is doubled, then the force of gravity is doubled. However, if the distance is doubled, then the force is four times weaker. This notion is a famous mathematical relationship that resulted from Newton’s work called the inverse square law. His theory of gravity is explained mathematically in the equation:
F=G Mm/r^2
where G is the gravitational constant that came about as a result of experimental data. M and m are the masses of the two objects, and r is the distance between the two objects.
Despite the triumph of Newton’s theory of gravity, as technology progresses over time, physicists have launched satellites and telescopes into space to obverse astronomical conditions. As a result of this, new astronomical data contradicts Newton’s equation. The strength of gravity does not exactly follow the dictation of the equation. Instead of a nice curved graphic representation of strength over distance, the graph expands over the regions forbidden by classical mechanics. Therefore, physicists had to come up with the idea of dark matter to resolve this conundrum. They believe that the entire universe is filled with dark matter that is responsible for the astronomical data of gravitational strength. However, besides this alternative, there is another approach to explaining the contradiction of Newton’s equation. This other approach is called Modified Newtonian Dynamics, or MOND. This idea was developed in the 1960s by a physicist from the Eastern hemisphere. He believes that the equation mistakenly neglected the inclusion of strong acceleration due to gravity. When Newton’s equation is applied on Earth, the gravitational acceleration is extremely small, so it is accurate to predict the force of gravity. MOND simply “modifies” the equation to fit experimental data. However, this idea is not as widely accepted as dark energy and dark matter because dark energy and dark matter could be used to explain so much more. However, someday, if dark matter fails, MOND will be there to account this problem.