In a hypothetical univers
In a hypothetical universe where quark masses are made very large but the QCD scale unchanged, this would become the lightest meson, because the masses of the mesons we know would scale like quark masses.  Attempts to calculate its properties have made use of the AdS/CFT duality, or of lattice QCD.  Experimentally it has remained elusive, perhaps because it is a broad resonance and mixed with other mesonic states.  A theoretical proof of its existence in Yang-Mills would demonstrate a mass gap and win a million-dollar Clay Institute prize.  FTP, what is this theoretical color-singlet state made up of the force carriers of QCD, a bound state of gluons?
In a hypothetical universe where quark masses are made very large but the QCD scale unchanged, this would become the lightest meson, because the masses of the mesons we know would scale like quark masses.  
glueball