  We study a braneworld model in which a phantom-like behaviour occurs with
only cold dark matter and a cosmological constant, due to a large distance
modification of gravity. With the addition of curvature, the geometrical tests
are not strict enough to rule out models in which gravity is modified
significantly on large scales. We show that this degeneracy in the parameter
space is broken by the structure formation tests, such as the integrated
Sachs-Wolfe effect, which can probe general relativity on large scales.
