  The expansion of the Universe is accelerated as testified by observations of
SNeIa at varying redshifts. Explanations of this acceleration are of two kinds:
modifications of Einstein gravity or new forms of energy. An example of
modified gravity is the braneworld Dvali-Gabadadze-Porrati (DGP) model, an
example of dark energy is Chaplygin gas. Both are characterized by a cross-over
length scale $r_c$ which marks the transition between physics occurring on our
four-dimensional brane, and in a five-dimensional bulk space. Assuming that the
scales $r_c$ in the two models are the same, we study Chaplygin gas dark energy
in flat DGP geometries. The self-accelerating branch does not give a viable
model, it causes too much acceleration. We derive the Hubble function and the
luminosity distance for the self-decelerating branch, and then fit a
compilation of 192 SNeIa magnitudes and redshifts in the space of the three
parameters of the model. Our model with the self-decelerating branch fits the
supernova data as successfully as does the $\Lambda CDM$ model, and with only
one additional parameter. In contrast to the $\Lambda CDM$ model, this model
needs no fine-tuning, and it can explain the coincidence problem. It is unique
in the sense that it cannot be reduced to a cosmological constant model in any
other limit of the parameter space than in the distant future. If later tests
with other cosmological data are successful, we have here a first indication
that we live in a five-dimensional braneworld.
