  The current cosmological paradigm, LCDM, requires that the mass-energy of the
universe be dominated by invisible components: dark matter and dark energy. An
alternative to these dark components is that the law of gravity be modified on
the relevant scales. A test of these ideas is provided by the Baryonic
Tully-Fisher Relation (BTFR), an empirical relation between the observed mass
of a galaxy and its rotation velocity. Here I report a test using gas rich
galaxies for which both axes of the BTFR can be measured independently of the
theories being tested and without the systematic uncertainty in stellar mass
that affects the same test with star dominated spirals. The data fall precisely
where predicted a priori by the modified Newtonian dynamics (MOND). The scatter
in the BTFR is attributable entirely to observational uncertainty. This is
consistent with the action of a single effective force law but poses a serious
fine-tuning problem for LCDM.
