After a personal introduction, I give a brief and pedagogic introduction to a powerful tool, to the reduction mechanism which helps us to solve linear or non-linear partial differential equations or even systems in general. As a trivial example I show how it works on the regular diffusion equation  geting new kind of soutions even today. This theoretical background helped us to formulate a simple classical spherical symmetric hydrodynamical model which is capable to describe the hypothetical dark fluid. With additional cosmological considerations and a rotation term this model can explain the Hubble tension problem.