The nature of dark matter and dark energy is the most important topic in cosmology and one of the most important topic in physics. We have found plenty of evidence from observation, showing that the existence of dark matter and dark energy provides a simple and correct explanation for the observations. However, we almost know nothing about the nature of dark matter and dark energy. Is dark matter really composed of particles? Is dark energy just an illusion from modified gravity? We have too many questions about dark matter and dark energy. What can we do? Build models for the questions, predict what will be observed in such models and finally test the models with observations or experiments. This is the way of science.

Up to now, the most popular model for cosmology is the LCDM model. L represents Lambda, the cosmological constant, or dark energy. CDM means cold dark matter. This model contains only 6 free parameters, and the prediction of the model matches the observations quite well. Thus, LCDM model has been considered as the standard model of cosmology (though this standard model contains 95% weird dark things beyond the standard model of particle physics, laugh).