Many scientists are searching for particles even more fundamental than leptons, quarks, gluons, etc. and (from what I know) string theory tries to hypothesize one elementary "thing" that everything is made of. So is there a reason that we should think that our currently known elementary particles may not be the true elementary particles of the universe?
1$\begingroup$ Every time in the past that we sharpened our view on the world with a better microscope, a larger telescope or a higher energy particle accelerator, something new and unexpected popped up... why would it be different now? If anything, the overarching theme of science, at the moment is "You ain't seen nothing, yet!". Concrete evidence that we don't know enough comes from both experiment and theory, most importantly, we have no clue what gravity does at short distances/high energies and then there is this pesky stuff called "dark matter". :-) $\endgroup$– CuriousOneJul 5, 2016 at 22:32
1$\begingroup$ Disregarding gravitation, we still don't know what dark matter is made of, and there's a technical thing called the strong CP problem that's unsolved in the standard model. We also don't really know why there is so much more matter than antimatter in the universe. If you search for "physics beyond the standard model" you will find resources about this, e.g. here on Wikipedia en.wikipedia.org/wiki/Physics_beyond_the_Standard_Model That article mentions neutrino masses by that is quite easily incorporated into the standard model and doesn't require new particles. $\endgroup$– Robin EkmanJul 5, 2016 at 22:36
1$\begingroup$ Another search term is the "hierarchy problem" en.wikipedia.org/wiki/Hierarchy_problem $\endgroup$– Robin EkmanJul 5, 2016 at 22:39
$\begingroup$ We can differentiate displacement as many times as we want but until it has no physical meaning it is just a problem of mathematics $\endgroup$– Mr. RobotJul 6, 2016 at 4:07
Some scientists do believe there are more fundamental particles (or strings, in the case of string theory). Others believe it isn't so much that there are more fundamental particles, it's that there are fundamental particles not yet discovered (as an obviously theoretical example, the graviton).
As for the reason, the current theory of physics (the Standard Model) doesn't explain nearly everything and has some problems! Examples (to name a few):
- it doesn't explain gravity
- doesn't address dark matter
- doesn't explain dark energy
- physicists call it inelegant: there are 19 "arbitrary" constants
- the hiearchy problem
- doesn't fit with a lot of discoveries in cosmology
I could go on (there are more here).
Finally, as CuriousOne said in the comments, every time we've sharpened our view on the world, something new and unexpected has popped up, from when Galileo first turned the telescope to the stars, providing proof of Copernicus' theory, to when Rutherford found evidence for antimatter in cloud chambers, to when two teams of physicists at CERN found evidence for a new particle that turned out to be the Higgs boson. Humanity is forever finding something new, and especially now, when we are finally exploring the outer reaches of the universe with LIGO and space telescopes, and probing the subatomic world with CERN, Fermilab, and others...there's no reason to think that will change.
Hope this helps!
$\begingroup$ The otHer big reason is that as far as experimental results we have only explored to about the TEV range. We know there is unknown physics in the Planck sizes/energies, because at least one known force, gravity, has to be brought in before then. That combined with the other problems cited by Heather, Robin and CuriousOne leads pretty much everyone to think there is new physics between 1 TEV and Planck. And, it turns out that higher energies is smaller sizes, so there's no way out but to look for the smaller sizes. Think of what we know as an effective theory, to be refined in the smaller sizes $\endgroup$– Bob BeeJul 6, 2016 at 0:49
$\begingroup$ Yes, and higher energy. That's why astrophysics and cosmology are finding some unexplained black matter-energy. We try to understand the large but high energy and the small which helps cause it and is also high energy. $\endgroup$– Bob BeeJul 6, 2016 at 0:56