What is the Equivalence Principle, per wikipedia:
https://en.wikipedia.org/wiki/Equivalence_principle

"In the theory of general relativity the equivalence principle is the equivalence of gravitational mass and inertial mass, and Albert Einstein's observation that the gravitational "force" as experienced locally while standing on a massive body (such as the Earth) is the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference."

It has been assumed by most people that whatever it is that gives matter the characteristic of inertia is also the thing that gives matter the characteristic of gravitational attraction. It has been assumed there is some intrinsic quality of matter that is equivalent for both gravity and inertia. Over time, in order to explain various phenomena in the universe, this eventually required the adoption of the idea that there is dark matter throughout the universe.

What is Dark Matter per Wikipedia: https://en.wikipedia.org/wiki/Dark_matter

"Dark matter is a form of matter thought to account for approximately 85% of the matter in the universe and about a quarter of its total mass-energy density or about 2.241×10⁻²⁷ kg/m³. Its presence is implied in a variety of astrophysical observations, including gravitational effects that cannot be explained by accepted theories of gravity unless more matter is present than can be seen."

But now there is evidence that dark matter might not exist and various phenomena in the universe might be explainable as coming from an "external field effect". This possibility was put forth by Mordehai Milgrom back in the 1980s and his theory is referred to as "Modified Newtonian Dynamics" (MOND), or "modified gravity". So now there is published evidence to support Milgrom's theory. This evidence was found by "an international group of scientists, including Case Western Reserve University Astronomy Chair Stacy McGaugh", Reference: https://phys.org/news/2020-12-unique-gravity-dark-theory.html

What is this External Field Effect per Wikipedia: https://en.wikipedia.org/wiki/
Modified_Newtonian_dynamics#The_external_field_effect

"In Newtonian mechanics, an object's acceleration can be found as the vector sum of the acceleration due to each of the individual forces acting on it. This means that a subsystem can be decoupled from the larger system in which it is embedded simply by referring the motion of its constituent particles to their centre of mass; in other words, the influence of the larger system is irrelevant for the internal dynamics of the subsystem. Since Milgrom's law is non-linear in acceleration, MONDian subsystems cannot be decoupled from their environment in this way, and in certain situations this leads to behavior with no Newtonian parallel. This is known as the "external field effect" (EFE).^[1]"

That is all well and good but it doesn't really explain what the external field really is or how matter generates this field. Ernst Mach is the first person this author knows of to propose an external field effect. Einstein called this "Mach's Principle". What is Mach's Principle per Wikipedia: https://en.wikipedia.org/wiki/Mach's_principle

"In theoretical physics, particularly in discussions of gravitation theories, Mach's principle (or Mach's conjecture^[1]) is the name given by Einstein to an imprecise hypothesis often credited to the physicist and philosopher Ernst Mach. The purpose of the hypothesis was to attempt to explain how rotating objects maintain a frame of reference, such as gyroscopes and spinning celestial bodies. The idea is that the existence of absolute rotation (the distinction of local inertial vs. rotating reference frames) is determined by the large-scale distribution of matter, as exemplified by this anecdote: ^[2]

You are standing in a field looking at the stars. Your arms are resting freely at your side, and you see that the distant stars are not moving. Now start spinning. The stars are whirling around you and your arms are pulled away from your body. Why should your arms be pulled away when the stars are whirling? Why should they be dangling freely when the stars don't move? "

Since, relatively speaking, you could be still and the universe spinning around you and still cause your arms to be pulled outward, Mach's Principle implies there is some kind of interaction between local mass and distant mass that can explain this force pulling on your arms. This force of inertia might be caused by something different than what causes the force of gravity. Bob Lazar is the first person this author knows of to put forth that there are multiple forces. Lazar described what he called "gravity A" and "gravity B". A few people reading this might remember this man, Bob Lazar, who claimed to have access to information about antigravity technology.

Who is Bob Lazar per Wikipedia: https://en.wikipedia.org/wiki/Bob_Lazar

"Robert Scott Lazar (/ləˈzɑːr/; born January 26, 1959) is an American conspiracy theorist who claims to have been hired in the late 1980s to reverse-engineer extraterrestrial technology at what he described as a secret site called "S-4". Lazar alleges that this subsidiary installation is located several kilometers south of the United States Air Force facility popularly known as Area 51."

It is this author's contention that people, not aliens, manufactured everything that Bob Lazar saw at Area 51 and he was unwittingly used to spread misinformation and disinformation about technology that was really developed right here on Earth. For example, aliens would have used a stationary phase array transmitting antenna for the "gravity emitters". But what Bob Lazar saw were mechanically pivoting emitters below the deck of the Sport Model. Back in the 1980's it would have been people, not aliens, who would have used relatively primative mechanically pivoting emitters instead of a non-moving phase array approach. Also, the Sport Model's landing legs and entry hatch were missing from the Sport Model that Bob had access to because people couldn't figure out how to make them look "alien" enough.

Getting back to the point: It is this author's contention there is some truth to what Bob Lazar said about gravity A waves and gravity B waves. He said gravity A waves are responsible for long range effects, in other words, the inertial force that all matter experiences and he said this comes from processes within the nuclei of atoms and is equivelant to the nuclear strong force. This author's research confirms this is likely as explained in the videos linked to below. Gravity B waves are responsible for what we normally call gravitational attraction that holds people to planets and keeps planets in solar systems, etc.. This gravitational force decreases with distance faster than the much longer range inertial force. It is this author's contention that these 2 forces come from 2 different but related phenomena. This is described in detail in a 3 part video uploaded to Youtube back in 2016 called "Where Gravity Comes From, Part 1 of 3" 2 of 3 and 3 of 3.

https://www.vasantcorporation.com/gravity-videos.php

It is the inertial force that supplies the "external field effect" that contributes to the total forces affecting the movement of matter in the universe. This inertial force and the gravitational force are Bob Lazar's Gravity A and Gravity B that together explain the motions of matter in the universe without the need for dark matter.

UPDATE: Please see the in-work presentation,
"Review of the Equivalence Principle"

Gravitational Mass versus Inertial Mass