Innovative experimental techniques are needed to further search for dark matter weakly interacting
massive particles. The ultimate limit is represented by the ability to efficiently reconstruct and
identify nuclear and electronic recoil events at the experimental energy threshold. Gaseous Time
Projection Chambers (TPC) with optical readout are very promising candidates thanks to the 3D
event reconstruction capability of the TPC technique and the high sensitivity and granularity of
last generation scientific light sensors. The CYGNO experiment is pursuing this technique by
developing a TPC operated with He-CF$_4$ gas mixture at atmospheric pressure equipped with a Gas
Electron Multipliers (GEM) amplification stage where visible light is produced. The combined
use of high-granularity sCMOS cameras and fast light sensors allows the reconstruction of the 3D
direction of the tracks, offering good energy resolution and very high sensitivity in the few keV
energy range, together with a very good particle identification useful for distinguishing nuclear
recoils from electronic recoils. We present the design and the sensitivity of a 50L prototipe which
is currently being installed underground at LNGS and will be operated already in 2022. The
performances of the prototype are evaluated with an advanced Monte Carlo simulation and by
calibration runs with radioactive source. We show that good energy and spatial resolution as
well as discriminating power between nuclear and electronic recoils is achieved in the keV energy
range. The Cygno collaboration plans to demonstrate the scalability of such detector concepts to
reach a target mass large enough to significantly extend our knowledge about DM nature and solar
neutrinos.