- We report the discovery in space of a disilicon species, SiCSi, from observations between 80 and 350 GHz with the IRAM 30 m radio telescope. Owing to the close coordination between laboratory experiments and astrophysics, 112 lines have now been detected in the carbon- rich star CW Leo. The derived frequencies yield improved rotational and centrifugal distortion constants up to sixth order. From the line profiles and interferometric maps with the Submillimeter Array, the bulk of the SiCSi emission arises from a region of 6''in radius. The derived abundance is comparable to that of SiC 2. As expected from chemical equilibrium calculations, SiCSi and SiC 2 are the most abundant species harboring a Si− C bond in the dust formation zone and certainly both play a key role in the formation of SiC dust grains.