Experiment after experiment has tried to find flaws in the Standard Model's predictions, but so far all the experimental evidence supports it. Nevertheless, scientists do not believe that the Standard Model provides complete answers to all our questions about matter.
It describes everything we see in the laboratory. Aside from leaving gravity out, it's a complete theory of what we see in nature. But it's not an entirely satisfactory theory, because it has a number of arbitrary elements. For example, there are a lot of numbers in this standard model that appear in the equations, and they just have to be put in to make the theory fit the observation. For example, the mass of the electron, the masses of the different quarks, the charge of the electron. If you ask, "Why are those numbers what they are? Why, for example, is the top quark, which is the heaviest known elementary particle, something like 300,000 times heavier than the electron?" The answer is, "We don't know. That's what fits experiment." That's not a very satisfactory picture.
When you look even closer there are many things wrong with this model. Some of these include:
- Gravity: Most important of all. Where the hell's gravity? A theory of quantum gravitation, or more formally quantum geometrodynamics, does not yet exist. Incorporating gravity into particle physics looks to be a horrendous challenge.
- Arbitrary parameters (like the mass of the electron)
- Planck Limits: The Standard Model describes quite accurately physics near the electroweak symmetry breaking scale (246 GeV). But the Standard Model is only a "low energy" approximation to a more fundamental theory. The Standard Model cannot be valid at energies above the Planck scale (~1019 GeV), where gravity can no longer be ignored.
- Cosmology: dark matter and dark energy. Cosmological observations tell us the standard model explains about 5% of the energy present in the Universe. About 26% should be dark matter, which would behave just like other matter, but which only interacts weakly (if at all) with the Standard Model fields. Yet, the Standard Model does not supply any fundamental particles that are good dark matter candidates. The rest (69%) should be dark energy, a constant energy density for the vacuum. Attempts to explain dark energy in terms of vacuum energy of the standard model lead to a mismatch of 120 orders of magnitude.
- Matter-antimatter asymmetry: the Universe is made out of mostly matter. However, the standard model predicts that matter and antimatter should have been created in (almost) equal amounts if the initial conditions of the Universe did not involve disproportionate matter relative to antimatter. Yet, no mechanism sufficient to explain this asymmetry exists in the Standard Model.
Once we decide to tackle gravity, the Standard Model as we know it transforms beyond recognition and an ultimate Theory of Everything becomes possible. We could then say that physics has reached its end.