The coincidence problem asks why the energy density of dark energy and the energy density of matter (including dark matter) are comparable in magnitude today, despite evolving very differently throughout cosmic history. In Lambda-CDM, dark energy (the cosmological constant) has been essentially constant since the Big Bang, while matter density decreases as the universe expands. The probability that these two independent components would happen to be nearly equal precisely in the present epoch—after billions of years of divergent evolution—appears astronomically small and requires inexplicable fine-tuning. This coincidence suggests either that our current understanding of dark energy and matter is incomplete, or that some deeper principle connects them (Steinhardt et al. 1999; Carroll et al. 2004). SCT must explain why dark energy and matter densities are comparable today without invoking fine-tuned initial conditions or unknown coupling mechanisms.