The paper by Ashoke Sen explores the classical decay of unstable D-branes in both bosonic and superstring theories, demonstrating that this decay produces a pressureless gas with non-zero energy density. This energy density is stored in the open string fields, even though there are no open string degrees of freedom around the minimum of the tachyon potential. The author provides a detailed analysis of the energy momentum tensor for rolling tachyon solutions in these theories and shows that the asymptotic form of the energy momentum tensor for large times is described by a pressureless gas. The energy density remains constant, while the pressure approaches zero as the tachyon field rolls towards its minimum. The paper also discusses the possibility of this tachyon matter system playing a role in cosmology, particularly in contributing to dark matter. Additionally, the author examines the nature of supersymmetry breaking induced by tachyon matter and the potential for other types of matter to arise from tachyon condensation in unstable brane systems. The analysis is supported by an effective field theory description, which confirms the observed behavior.The paper by Ashoke Sen explores the classical decay of unstable D-branes in both bosonic and superstring theories, demonstrating that this decay produces a pressureless gas with non-zero energy density. This energy density is stored in the open string fields, even though there are no open string degrees of freedom around the minimum of the tachyon potential. The author provides a detailed analysis of the energy momentum tensor for rolling tachyon solutions in these theories and shows that the asymptotic form of the energy momentum tensor for large times is described by a pressureless gas. The energy density remains constant, while the pressure approaches zero as the tachyon field rolls towards its minimum. The paper also discusses the possibility of this tachyon matter system playing a role in cosmology, particularly in contributing to dark matter. Additionally, the author examines the nature of supersymmetry breaking induced by tachyon matter and the potential for other types of matter to arise from tachyon condensation in unstable brane systems. The analysis is supported by an effective field theory description, which confirms the observed behavior.