The force that the standard model notoriously finds too challenging to describe is the very first force we learn about at school, gravity. Our best theory of gravity is Einstein's general relativity. Unifying the current standard model with general relativity would successfully create a Theory of Everything.
It seem rather simple then, doesn't it, to fulfil the physicist's dream? However, the current standard model and general relativity have proven horrendously difficult to unify, with attempts resulting in 'complete nonsense'. Physicists have turned their hopes onto other theories (including the commonly cited string theory), in the hope that these new routes will be more fruitful.
There, for now, the standard model remains: alongside Einstein's general relativity as one of two theories almost everything.