Observations of galaxies and galaxy clusters in the local universe can
account for only $10\%$ of the baryon content inferred from measurements of the
cosmic microwave background and from nuclear reactions in the early Universe.
Locating the remaining $90\%$ of baryons has been one of the major challenges
in modern cosmology. Cosmological simulations predict that the 'missing
baryons' are spread throughout filamentary structures in the cosmic web,
forming a low density gas with temperatures of $10^5-10^7$ K. Previous attempts
to observe this warm-hot filamentary gas via X-ray emission or absorption in
quasar spectra have proven difficult due to its diffuse and low-temperature
nature. Here we report a $5.1 \sigma$ detection of warm-hot baryons in stacked
filaments through the thermal Sunyaev-Zel'dovich (SZ) effect, which arises from
the distortion in the cosmic microwave background spectrum due to ionised gas.
The estimated gas density in these 15 Megaparsec-long filaments is
approximately 6 times the mean universal baryon density, and overall this can
account for $\sim 30\%$ of the total baryon content of the Universe. This
result establishes the presence of ionised gas in large-scale filaments, and
suggests that the missing baryons problem may be resolved via observations of
the cosmic web.