The dominant and most mysterious ingredient of the current standard model, is, without doubt, the so-called dark energy, that contributes about 75% of the mass/energy density of the Universe, and causes the current accelerating expansion. The most direct evidence for this acceleration comes from the observation of "standard candles" at cosmological distances, or rather objects whose intrinsic luminosities can be calibrated a priori, so that their distance can be measured via their apparent luminosity. The so-called type Ia supernovae have been used in this way, and at the end of the '90s allowed the acceleration to be detected for the first time. Current and future research will concentrate on identifying a time dependence on the so-called "equation of state" of dark energy, to demonstrate whether dark energy is simply Einstein's cosmological constant, or if it is connected to a primordial energy field also related to the formation of elementary particles. To this end, various projects either underway or proposed for the future, are dedicated to systematic searches for type Ia supernovae, both from the ground and space. These will surely greatly increase the volume of data on distant SNe, shedding new light on the nature of cosmic acceleration. The search for and the calibration of new standard candles (such as "core-collapse" SNe and gamma-ray bursts) are also areas of research receiving growing attention.

Other experiments plan to cover large areas of the sky with high quality images (and so in general observed from space) to measure so-called weak lensing. This distortion, introduced in the images of "background" galaxies by the mass distribution along the path that the photons travel to reach us, depends on the gravitational growth of fluctuations, and ultimately on the cosmological parameters. The final goal of these surveys is make maps of the "shear", as the weak lensing deflection is called, in various redshift intervals and reproduce the tomographic distribution of cosmic structure at successive epochs. These "tomographic plates" directly measure the expansion and structure growth history of the Universe that depend directly on the presence of dark energy.

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