  We study the properties of voids in two different types of coupled scalar
field theories. Due to the fifth force produced by the scalar field coupling,
the matter particles feel stronger attraction amongst each other and cluster
more quickly than they do in the standard LCDM model. Consequently voids in the
coupled scalar field theories start to develop earlier and end up bigger, which
is confirmed by our numerical simulations. We find that a significantly larger
portion of the whole space is under-densed in the coupled scalar field theories
and there are more voids whose sizes exceed given thresholds. This is more
prominent in early times because at later times the under-dense regions have
already been evacuated in coupled scalar field theories and there is time for
the LCDM model to catch up. The coupled scalar field theories also predict a
sharper transition between voids and high density regions. All in all, the
qualitative behaviour is different not only from the LCDM result, but also
amongst specific coupled scalar field models, making voids a potential
candidate to test alternative ideas about the cosmic structure formation.
