In a few dozen seconds, gamma ray bursts (GRBs) emit up to ~10⁵⁴ erg in terms of an equivalent isotropically radiated energy E _iso , so they can be observed up to z ~ 10. Thus, these phenomena appear to be very promising tools to describe the expansion rate history of the universe. Here, we review the use of the E_p,i–E _iso correlation of GRBs to measure the cosmological density parameter Ω_M. We show that the present data set of GRBs, coupled with the assumption that we live in a flat universe, can provide independent evidence, from other probes, that Ω_M ~ 0.3. We show that current (e.g. Swift, Fermi/GBM, Konus-WIND) and forthcoming gamma ray burst (GRB) experiments (e.g. CALET/GBM, SVOM, Lomonosov/UFFO, LOFT/WFM) will allow us to constrain Ω_M with an accuracy comparable to that currently exhibited by Type Ia supernovae (SNe–Ia) and to study the properties of dark energy and their evolution with time.