/usr/include/vtk-5.10/Cosmo/ is in libvtk5-dev 5.10.1+dfsg-2.1build1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/vtk-5.10/Cosmo/BasicDefinition.h
/usr/include/vtk-5.10/Cosmo/ChainingMesh.h
/usr/include/vtk-5.10/Cosmo/CosmoDefinition.h
/usr/include/vtk-5.10/Cosmo/CosmoHalo.h
/usr/include/vtk-5.10/Cosmo/CosmoHaloFinder.h
/usr/include/vtk-5.10/Cosmo/CosmoHaloFinderP.h
/usr/include/vtk-5.10/Cosmo/FOFHaloProperties.h
/usr/include/vtk-5.10/Cosmo/HaloCenterFinder.h
/usr/include/vtk-5.10/Cosmo/Message.h
/usr/include/vtk-5.10/Cosmo/ParticleDistribute.h
/usr/include/vtk-5.10/Cosmo/ParticleExchange.h
/usr/include/vtk-5.10/Cosmo/Partition.h
/usr/include/vtk-5.10/Cosmo/winDirent.h