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With what we know about physics, is it possible that when the universe 'began', around when quarks and leptons were produced, another particle, which doesn't couple to either quarks, leptons or photons was also produced ? The only other way that we can observe its existence is via the effects of its gravitational field. In others words, some ''dark-matter-particle'' that doesn't interact with known forms of matter, except through gravity?

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Although we don't know what dark matter is, most theories have it as a WIMP, a weakly interacting massive particle. Thus it would interact with the weak force as well as gravity. –  jhobbie Jul 18 at 3:07
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@jhobbie WIMPs are popular (at least partly) because we know how to look for them (and because they can cool; I suspect that the OP's guess won't cool right), but this is mostly a matter of "looking for our keys under the streetlight". If we don't even know how to try to detect a particular flavor of dark matter than there is little point in putting a lot of effort into it. –  dmckee Jul 18 at 3:17

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This answer is within the current physics and theoretical understanding, which has developed a successful formalism that includes all the experimentally seen particles in the Standard Model. The model has been very successful in predicting several new particles using its symmetry and mathematics, the experimental observation of the Higgs boson serving as perhaps the last point of validation.

This model does not include gravity and theorists are working hard to form a model that will encompass the standard model's $SU(3) \times SU(2) \times U(1)$ group structure, in more complicated models that include the quantization of gravity. String models promise to be able to do so but the theories have not advance to the point of proposing a solid model from the thousands of possibilities. Once such a model is proposed it can be checked on whether there exist predictions for particles that to first order interact only with the gravitational field.

There are proposals to see the results of graviton resonances at the LHC, and it might be that in some string model a first order stable or long living gravitation only particles will appear in the final formulation. Certainly though the particle would interact in higher orders with other particles within the standard formulations of the theory.

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Yes, there have been suggestions that such particles exist, and an example is the sterile neutrino.

But your question is a little more involved than you might think at first sight. For example if the sterile neutrino only interacts through gravity what interaction caused it to be created in the first place? There is nothing in the Standard Model that could create such particles. However we expect that the Standard Model is a low energy approximation and as we work backwards in time towards the Big Bang and the energies get higher we'll need a grand unified theory like SO(10) and ultimately a quantum theory of gravity (which may or may not be String Theory). These contain interactions that can create particles like sterile neutrinos. However this remains a speculative area of Physics and at the moment we can't say definitely whether such particles exist or if they exist how they were created.

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Hi John, this was a good answer. Under the assumption that some sort of quantum gravity is the correct explanation of the universe, couldn't the particles have been made via some gravitational QFT? Is this just too speculative to really consider yet? –  Brandon Enright Jul 18 at 6:04
    
@BrandonEnright: For sterile neutrinos I believe there are interactions in SO(10) that can create them so you don't need gravity. I way out of my depth though, so don't take this as gospel. String theory predicts all sorts of particles of this type. They are generically known as hidden sectors. –  John Rennie Jul 18 at 6:25
    
Thanks for the reply. I hadn't heard of these other possible interactions before so thank you. Am I reading too much into your response though by thinking that creating particles other than gravitons with some sort of quantum gravity interaction is problematic? Could stable, massive particles that interact only via gravity be (realistically) created or are there just too many problems (or unknowns) with that? I know this follow-up might be too much for comments so don't feel too obliged to reply :-) –  Brandon Enright Jul 18 at 6:58
    
Nice answer, but please correct me if I'm mistaken. As regards "... nothing in Standard Model that could create such particles", we need to go beyond the Standard Model even to account for baryo-genesis if I'm right? And unlike WIMPs, we know for sure that baryons exist! So, the model is clearly unsatisfactory as regards the 'genesis' part is concerned, (except for composites). –  New_new_newbie Jul 18 at 7:00
    
@New_new_newbie: yes, it's generally accepted that the Standard Model must be replaced by some more general theory at high energies. The only question is what replaces it, and that isn't currently known. –  John Rennie Jul 18 at 7:02

Let us clearly draw the line between two things here, since the question can easily involve opinion based answers, which may also be dubbed non-mainstream (which isn't welcome on this site).

1) The existence of dark matter is generally believed by a majority of the Physics community, since astronomical observations, notably by the Planck space observatory, suggest that visible matter makes up only $\sim 5 \%$ of the total mass energy of the universe. Most conveniently, that means that some matter isn't visible to us, hence we call it 'dark', and look at extensions of current theoretical frameworks to account for them, as John and Anna have mentioned in their answers.

However, there are alternatives, which are intended towards accounting for these observations without including any extra invisible matter, such as the Modified Newtonian Dynamics Approach. If this is proved correct with time, we won't need to worry about the dark-matter particles you are asking about. The final answer is really far off, since neither of these mentioned approaches can claim to be the final word on this subject.

2) Regarding ''Dark Matter Particles'', as dmckee mentions in a comment, one can't be absolutely sure as to how to look for them with the current set of theoretical understanding, yet I am aware of certain experimental groups having reported measurements relating to WIMPs, such as the widely popularized Super Cryogenic Dark Matter Search (SuperCDMS) experiment. The strategy here is to measure the recoil energy of a nucleus in a Nucleus-WIMP collision, which is more or less a direct detection as compared to the indirect method - analyzing the debris in WIMP decays or annihilations. They had reported some measurements, at roughly $3\sigma$ level, which are encouraging signs, though not conclusive yet. (An alternative reference can be found here ). (Sidenote - The trust level is a little higher than $3\sigma$; nobody believed the Higgs boson ''discovery'', till a $5\sigma$ measurement was reported.)

A completely consistent theory of their genesis, is an issue that can be settled only after we know for sure that they exist. Right now, neither the theoretical nor the experimental status of this problem can warrant that. Or worst case, if MoND works out fine, we may not even need them. Only time will tell.

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Thank you all for answering my question. What I'm really trying to get at is the following question. Similar to how quarks and leptons are created and the fields of the photons, weak bosons, gluons, and Higgs, could a particle also have been made that only interacts with gravity? Meaning we only observe it by its gravity. Basically the simplest particle that you can think of being produced that could explain all the effects that we attribute to dark matter. As mentioned maybe this awaits a theory of quantum gravity, assuming there is one. –  Budnpk Jul 18 at 23:51
    
sterile neutrinos might be it, if so how could we detect them? –  Budnpk Jul 20 at 17:52
    
@Budnpk - I suppose something related to that is mentioned here, in the link that John shared. Neutrino detection is prickly anyways, due to their small interaction cross section. –  New_new_newbie Jul 21 at 8:32

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