The dark matter can be a gravity effect that forms without mass.

The key question in dark matter is, what put gravity fields into motion? Some researchers suggest that the changes in spacetime curvature are the things that make gravity waves move. 

A new theory suggests that gravity is possible without mass. And that is an exciting new model for dark matter. Researcher Richard Lieu from Alabama State University suggests that dark matter is a distortion in 3D spacetime. 

In this model, the third dimension is like the quantum jelly or a very large, and thick wave movement. Those waves' thickness is so large, that the entire universe fits in it. If there are lower energy fields in some areas, that can make energy travel in them. In some models, the electromagnetic radiation can turn straight to the other type of wave movement. 

If counterwaves impact those electromagnetic- or quantum fields they can stretch or gather those waves. That means there is the possibility that impacting waves can turn radio waves or gamma rays into gravity waves. The idea is that impacting waves adjust the wave movement's wavelength. Theoretically is possible that this thing can transform X- or gamma rays into the gravity waves. 

The idea is that the shape of the 3D spacetime can change in a vacuum, and that can form the condition that causes the effect, that looks like gravity. When we think about dark matter, we normally mean the mysterious gravitational effect. And normally when we talk about wave movement, that transports gravity. 

So, dark matter is the thing that makes gravity waves travel. And the thing that can move the gravity field, or form the gravity waves is the lower energy gravity field. Gravity is like all other wave movements or energy forms. Gravity waves always travel to the lower energy gravity field. And it should behave like all other wave movements. 

Dark matter is the effect that moves gravity waves. The thing. What makes this material dark is that we cannot see the thing, which makes those gravity waves move. Standing energy fields don't make a thing. Only moving gravity waves can cause the gravity effect. So when we think about situations where the gravity waves impact, that case can form gravity wave "hills" that send gravity waves around the universe. 

https://academic.oup.com/mnras/article/531/1/1630/7673084