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., Phys. Rev. Lett. 87, 216402 (2001)], and the excitation was assigned, in spite of the 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.