Schematic drawing of compression of a dust aggregate consisting of a number of BCCA clusters. The left figure shows a dust aggregate consisting of many BCCA clusters and the BCCA clusters are distributed three dimensionally. Each enclosed line represents each region dominated by the BCCA clusters. The central figure is a BCCA cluster, receiving pressure from the next clusters. The BCCA cluster has a large void depicted in the central figure, and thus the void would be compressed, as expressed in the right figure. The required energy to compress the void is the energy to rotate the connection of monomers in contact. Therefore, the compression can be determined by the rolling motion of monomer connection on the connecting point of the subclusters.