  New exact solutions of brane-world cosmology are given. These solutions
include an arbitrary constant C, which is determined by the geometry outside
the brane and which affects the cosmological evolution in the brane-world. If C
is zero, then the standard cosmology governs the brane-world as a low-energy
effective cosmological theory. However, if C is not zero, then even in
low-energy the brane-world cosmology gives predictions different from the
standard one. The difference can be understood as ``dark radiation'', which is
not real radiation but alters cosmological evolutions.
