Four fundamental forces explain the inner workings of the universe: The strong nuclear force, the weak nuclear force, the electromagnetic force, and the gravitational force. Their influence on the behavior of the universe is predictable and can be directly observed. Dark matter, however, is a mass that we can’t see, but we know it’s present as we can observe the changes in the behavior of planetary orbits and gravitational lensing that the four fundamental forces cannot cause. These anomalies observed in the universe can only be explained through the existence of dark matter. There are two leading theories for dark matter: WIMPS and MOND. 

1. WIMPS

WIMPS, or Weakly Interacting Massive Particles, are particles that interact with weak nuclear force and gravity, these interactions make them a leading candidate for being Dark Matter. 

In this section, I will focus on WIMPs interactions with gravity, more specifically gravitational lensing – a compelling argument for its existence. Gravitational lensing is a phenomenon where light bends around bodies of mass to reach the observer. Physicists see evidence of dark matter clusters having this lensing effect on light as well. So if WIMPs exist, they can be attributed to this phenomenon as they share the same properties as theorized dark matter: they don’t emit electromagnetic radiation, demonstrating a weak interaction with matter, but more importantly WIMPs have mass and anything with mass has gravity. This scenario demonstrates a perfect match: an undetectable gravitational influence on the universe. 

Dark Matter doesn’t produce electromagnetic radiation and as stated before, physicists can only detect dark matter in the universe through its interactions with gravity as it doesn’t emit electromagnetic radiation. However, physicists have discovered that when WIMPs annihilate each other, their mass converts to energy, emitting high-energy particles in the form of gamma rays – a type of electromagnetic radiation. 

This discovery doesn’t contradict the idea that dark matter is undetectable. WIMPs, in their normal state, remain undetectable through electromagnetic interactions as they do not emit electromagnetic radiation. However, when they annihilate, they emit gamma rays. In fact, if WIMPS are the explanation for dark matter, detecting the electromagnetic radiation emitted after annihilation is a gateway to discovering the behavior of dark matter more directly. Nonetheless, these annihilation occurrences are extremely infrequent and it is WIMPs’ weak nuclear interactions and ability to account for observed gravitational effects make them a leading candidate for dark matter.

*https://svs.gsfc.nasa.gov/10955#:~:text=Weakly%20Interacting%20Massive%20Particles%2C%20or,annihilate%20and%20make%20gamma%20rays.

Intuitive Animation about WIMPs created by NASA

2. MOND (in the context of Dark Matter)

MOND, Modified Newtonian Dynamics, postulates there is no such thing as dark matter. And says the reason for these gravitational anomalies is because Newton was wrong about Gravity. His equation, F_g = G(m_{1 *} m₂)/d², where: F_g = The magnitude of the gravitational force, G = Gravitational constant, m₁ & m₂ = Two masses, d = distance, says that the force of gravity is dependent on the product of the two masses, and inversely dependent on the square distance between them. The reason behind why some physicists think he is wrong about this equation is because his laws of gravity are not accurate at extremely low acceleration – think weak gravitational fields or extremely large distances – where General Relativity and MOND provide more accurate equations.

It is exciting to see development in this field and as always keep exploring the universe!