Glueballs, predicted "bound" states consisting only of gluons, have been predicted using approximations to the Standard Model, but have never been observed. If they existed they would be very massive and have an extremely short lifetime.

Largely ignored by textbooks, there have been a huge number of different so-called “bag models,” of the nucleon and its excited states, proposed since the 1970s. All the models confine three free valence quarks inside a surface  or spherical box of some kind, with various different boundary conditions at, and couplings to, the surface. People are still working on various complex bag systems, but it is hard to see how any real basic physical insight can result, although various basic symmetry principles can be applied in creating a bag system.  All bag models stem from a very simple model originally suggested by Bogoliubov in 1967.  Research using bag models continues even today (2022), with the latest wrinkles being chiral bag models, and bag models for mesons. 

The so-called Standard Model consists of Quantum Chromodynamics, the theory of the strong interaction, and the Electroweak Theory, the theory of electromagnetic and weak processes. No flaw in the model has yet shown up; experimental results support the Standard Model to high precision. However, one obvious problem is that it contains 26 parameters that must be put in "by hand," based on experimental results, with no justification from some deeper theory.  Parameters of the Standard Model.