The present invention relates to composite particles containing silicon and carbon, wherein a domain size region of vacancies of 2 nm or less is 44% by volume or more and 70% by volume or less when volume distribution information of domain sizes obtained by fitting a small-angle X-ray scattering spectrum of the composite particles with a spherical model in a carbon-vacancy binary system is accumulated in ascending order, and a true density calculated by dry density measurement by a constant volume expansion method using helium gas is 1.80 g/cm3 or more and 2.20 g/cm3 or less.