Physics allows a scientist to predict the future. Having understood by what law a particular process develops, one can say for sure what will happen to the object after some time. It would seem that this is the most powerful tool in the hands of man! But no: mathematics is much more interesting, because it helps to outstrip any physical experiments by tens of years, predicting what has not yet been discovered. Like hypothetical particles.
The answer to the question lies on the surface: a hypothetical particle is one that has not yet been discovered - has not been discovered or registered. Until recently, this was, for example, the Higgs boson. But the question arises: where did such a concept come from, if in practice no one came across it even close?
So, modern physics "stands" on the quantum field theory, from which the physics of elementary particles follows. In essence, science is based on the thesis that everything in the universe consists of pieces so small that it is impossible to divide them into anything else. At the same time, the particles have completely different properties and nothing can unite them with each other.
Everything can be conditionally divided into two groups: "substance" and "interaction". If there are no questions with the first, then the second is an attempt at the most fundamental level to explain where gravity, magnetism and other forces come from. It is important to note that already at this stage, all science goes into a purely mathematical apparatus, very weakly amenable to experiments.
The passion of scientists is to simplify as much as possible by connecting all the elements together - an example of this is supersymmetry. This is a theory (hypothetical, i.e. unproven) that unites matter and interaction into one system, allowing one to transform one particle into another (in fact, to make matter from pure energy).
In the depths of this theory, hypothetical particles are born. At the mathematical level, each particle we know is associated with a “supersymmetric partner”: i.e. something the same, but with a minus sign. In particular, it is these elements that would make up "dark matter", the existence of which has also been proven only at the level of mathematical theory.
In the general case, more than a dozen more particles can be considered "hypothetical" (like a graviton, which would explain gravitational interactions) - but this concept is somewhat broader.