Why people think dark matter as a massive particle? Couldn't it be gravitational waves?

Dark matter is said to be present in the galaxies which helped galaxies to exist in the first place forming the glue to attach all the matter in the galaxies.

Given that dark matter never been seen or detected and only effect of which could be felt why do we believe it would be a particle? We have universe (almost) equally distributed in all directions with dark matter forming the skeleton for all the matter that could be seen. Can the dark matter be a illusion of gravitational waves interfering with each other forming this uniform interference pattern of galaxies/matter that is seen by us?

That would also explain the repulsive dark energy which would be the troughs in the gravitational waves indicating desructive interference in the gravitational waves.

Can we hope to simulating the above to come to the possible conclusion of source(two or many) of these gravitational waves that could give rise to skeleton of dark matter and dark energy which may be present in some higher dimentions?

• I don't really understand what you're saying, but you might be proposing something along the lines of Modified Newtonian Dynamics. Jun 21, 2017 at 2:20
• As I read through some parts of the above page it talks about the how newtonian laws behaving differently in rotatting galaxies where we expect outer spirals to rotate slowly but observation shows different results, so modification in these laws are proposed. But I am asking about the uniform skeleton of dark matter may have been formed by gravitational wave interference where constructive interference created the galaxies(with illusive dark matter) and destructive interference gives illusion of dark energy. Jun 21, 2017 at 2:33
• That's just one popular theory. There are still possible MACHO theories, I think. This video gives a good overview. Jun 21, 2017 at 3:57
• I'm not entirely sure I understand what you're getting at. There is no mechanism that I know of whereby gravitational waves can cause a consistent expansion of the metric of space. In any case, to get such large effects we'd be in the realm of non-linear gravity, where we can no longer talk about constructive and destructive interference of gravitational waves because they behave in a non-linear way. Any such model would be unacceptably non-parsimonious Jun 21, 2017 at 4:13
• @Chuxley: I am not an expert, but if gravity wave source(s) itself are moving shouldnt it cause illusion of expansion? Also regarding nonuniformity of dark matter, as far as I know we map dark matter exist by feeling the effect of light of distant star reaching to us through "clouds" of dark matter. so this map is within the galaxy where it may be non uniform. But if we consider dark matter forming the skeleton of all the matters/galaxies, we can assum dark matter also spread uniformly considering all the matter spread out uniformly. Jun 21, 2017 at 4:28

What we know about dark matter is that it exerts a gravitational pull, or in general relativity (GR) it acts as otherwise non-interacting matter. We've mapped it's density (in terms of say equivalent grams/$cm^3$), and we observe that it tends to be much higher where there is also other matter nearby, such as in the halo of galaxies, and much less where we also see few galaxies. The mapping of where we find it is consistent with it being matter that however has no electromagnetic nor strong nuclear force interactions. It shows gravitational interactions, just like any mass would. It may interact through the weak nuclear force, but it'd be too weak to observe.

We've mapped it for instance in the Bullet Cluster, see the Wikipedia article and picture at https://en.m.wikipedia.org/wiki/Bullet_Cluster. It shows two groups of galaxies passing through each other, and being slowed down by interactions, but the dark matter is not affected so we know it interacts little with itself or other matter, except gravitationally. We know a lot of things about it. The Bullet Cluster is a great example of why it's got to be different than normal matter, but yet always close to high densities of normal matter.

We know it can't be pure energy, i.e., zero mass particles going at the speed of light, because we see it going along and being around galaxies, and if it was zero mass non interacting particles it would just dissipate and separate from the galactic visible matter.

It cannot be electromagnetic waves as it would interacts with charged particles in many galaxies. It cannot be gravitational waves because again it would separate from the galactic matter.

Also note, gravitational waves peaks and troughs cannot represent either dark matter nor dark energy. The dark matter attracts itself so we see much more in certain areas. It is not interspersed with concentrations of dark energy, alternating or otherwise. Dark energy seems to be a constant density everywhere in the universe. They do not form patterns like waves with dark matter. There is no possible relationship.

This is the history of the universe at present, the present Big Bang model:

In particular, the quantum mechanical modeling before 10^-32 seconds, the inflation period, is proposed in order to generate the "skeleton" you are thinking about, i.e. explain the small, less than 10^-5 in-homogeneity observed in the cosmic microwave background , CMB, radiation. The quantum mechanical fliuctuations in the very early universe are the seeds where matter will converge by the time of the decoupling of photons, explaining the CMB plot..

By that time Newtonian physics should explain gravitational interactions locally following the General Relativity predictions globally.

So the skeleton is explained by quantum mecanical gravitational waves as they interact through the inflaton field in the very early universe. What is not explained without dark matter is the deviation of the rotational curves of the galaxies which indicate much larger concentrations of mass than the luminous mass.

There are alternative models proposed to weakly interacting particles to explain the necessity of non-luminous matter. Even alternative models that modify general relativity are proposed. It is an on going research. At the time of the history of the universe where the rotational curves are observed gravitational waves are very weak in the sense you imagine them to affect matter already existing in bulk.