  The Universe has evolved from an initial diffuse, uniform gas to a complex
structure that includes both voids and high-density galaxy clusters connected
by gaseous filaments, known as the Cosmic Web, and traced by 3D surveys of
galaxies. The filamentary structure contains a significant fraction of the
baryonic matter and is predicted to be mostly in the form of a moderately high
temperature plasma, the Warm Hot Intergalactic Medium. Plasma at this
temperature and ionization level emits mostly in soft X-rays. The filamentary
structure, however, is hard to detect because the other sources contributing to
the Diffuse X-ray Background are much brighter and, currently, there are very
few reported detections of emission from the filaments. We report the first
high-confidence level indirect detection of X-ray emission from the Warm Hot
Intergalactic Medium. Applying the Power Spectrum Analysis to XMM-Newton and
eROSITA data, we separated its contribution from other sources modeled in
previous studies. Our result is in good agreement with numerical simulations
and fills a critical gap in the picture of the large-scale structure of the
Universe, in which filamentary gas, galaxies and dark matter interact and
co-evolve.
