Future high-energy, multi-frequency survey astronomy, time-domain and multi-parameters inference, multi-messenger astro-particle physics, will not be able to grow in a really comprehensive, synergetic and serendipitous way, if an important, broad, electromagnetic window in the Universe, where interesting and multiple physical processes occur, is left unobserved. In this view e-ASTROGAM is a project of breakthrough space observatory mission, with large international interest, characterized by a detector composed by a silicon tracker, a calorimeter, and an anticoincidence system, dedicated to the, so called, poorly-known MeV-energy sky, through the detection of photons from about 150 keV to 3 GeV energy. e-ASTROGAM is optimized for simultaneous detection of Compton and pair-producing photon events, with optimal capabilities in gamma-ray continuum, nuclear line spectroscopy, spatial imaging, gamma-ray polarization measurement, and with a large field of view, essential for survey/time-domain multi-messenger synergy.
All these capabilities, will be able to open a rich, interdisciplinary, science menu in a electromagnetic band, the MeV Universe, probed only in a shallow way by previous satellites. e-ASTROGAM will be the ground-breaking instrument in case of nearby (about <20 Mpc) supernovae and kilonovae events, directly probing nucleosynthesis processes with a MeV line sensitivity one to two orders of magnitude better than previous/current instruments. The origin of key nuclear isotopes, responsible for the creation of about 50 percent the abundances of the nuclei heavier than iron, and for radioactive/chemical evolution of our Galaxy will be accurately determined. e-ASTROGAM is also a general-purpose satellite for broad and different scientific communities, with a rich scientific potential, placed in the enormous spectral energy vacuum between the multitude of soft-X-ray satellites (Athena for example) and the (>40 GeV) band of the Cherenkov Telescope Array (CTA). Powerful, multi-messenger experiments such as LISA, LIGO, Virgo, KAGRA, Einstein Telescope, Cosmic Explorer, IceCube-gen2, KM3NeT, SKA, ALMA, JWST, E-ELT, LSST, Athena, CTA will have a consistent leap in science results if coupled with e-ASTROGAM or similar all-sky/wide survey MeV space telescopes.