  4U 1820-30 is a low-mass X-ray binary near the center of the globular cluster
NGC 6624 consisting of, at least, one neutron star and one helium white dwarf.
Analyzing 16 years of data from the Rossi X-ray Timing Explorer (RXTE) allows
us to measure its orbital period and its time derivative with unprecedented
accuracy to be P = 685.01197 +- 0.00003 s and dP/dt /P = -5.3 +- 0.3x10^-8
yr^-1. Hence, we confirm that the period derivative is significantly negative
at the >17 sigma level, contrary to theoretical expectations for an isolated
X-ray binary. We discuss possible scenarios that could explain this
discrepancy, and conclude that the center of NGC 6624 most likely contains
large amounts of non-luminous matter such as dark remnants. We also discuss the
possibility of an IMBH inside NGC 6624, or that a dark remnant close to 4U
1820-30 causes the observed shift.
