  By direct absorption spectroscopy and comparison to photoluminescence (PL),
we investigate negatively charged excitons in dilute two-dimensional electron
systems at temperatures down to T=40 mK in the regime of the fractional quantum
Hall effect. At very low temperatures, for filling factor \nu < 1/3, an
additional excitation appears in the PL spectrum, between the well-known
singlet and triplet excitons. The observation of a similar excitation by PL was
reported very recently [G. Yusa et al., cond-mat0103561], and the excitation
was assigned, in spite of a PL intensity similar to that of the neutral
exciton, to be due to a dark triplet exciton. By comparing PL and direct
absorption spectra in optically thin samples at T<100 mK, we can identify the
new excitation indeed as a 'dark' mode, since we find that the oscillator
strength is much smaller than those of the 'bright' modes.
