The carbon dimer or dicarbon (C₂) has been known for nearly 150 years through its presence in comets, as initially reported by Donati in 1864, who observed the Swan bands that arise from an electronic exitation of C₂. Much later, Souza and Lutz reported the first detection of C₂ in an interstellar source in 1977. Their initial identification was made at the Smithsonian Institution's Mount Hopkins Observatory (now the Fred Lawrence Whipple Observatory) toward the hypergiant star Cygnus OB2-12. Since C₂ is symmetric and does not have a dipole moment, the detection was based upon the molecule's vibrational spectrum, using the Phillips bands. Some of the additional interstellar detections include: ζ Ophiuci by Chaffee and Lutz in 1978, ζ Persei and ο Persei by Hobbs in 1979 and 1981, respectively, and χ Ophiuchi, HD 154368, 147889, and 149404 by van Dishoeck and de Zeeuw in 1984.

The nature of the bonding in the ground state of C₂ continues to be debated. While the prevalent view is that it essentially has a double bond (mostly due to pi bonding), an argument has been advanced by Shaik et al. that C₂ has a quadruple bond.