  We present the first identification in interstellar space of the propargyl
radical (CH2CCH). This species was observed in the cold dark cloud TMC-1 using
the Yebes 40m telescope. The six strongest hyperfine components of the
2,0,2-1,0,1 rotational transition, lying at 37.46~GHz, were detected with
signal-to-noise ratios in the range 4.6-12.3 sigma. We derive a column density
of 8.7e13 cm-2 for CH2CCH, which translates to a fractional abundance relative
to H2 of 8.7e-9. This radical has a similar abundance to methyl acetylene, with
an abundance ratio CH2CCH/CH3CCH close to one. The propargyl radical is thus
one of the most abundant radicals detected in TMC-1, and it is probably the
most abundant organic radical with a certain chemical complexity ever found in
a cold dark cloud. We constructed a gas-phase chemical model and find
calculated abundances that agree with, or fall two orders of magnitude below,
the observed value depending on the poorly constrained low-temperature
reactivity of CH2CCH with neutral atoms. According to the chemical model, the
propargyl radical is essentially formed by the C + C2H4 reaction and by the
dissociative recombination of C3Hn+ ions with n = 4-6. The propargyl radical is
believed to control the synthesis of the first aromatic ring in combustion
processes, and it probably plays a key role in the synthesis of large organic
molecules and cyclization processes to benzene in cold dark clouds.
