We have revealed the overall porosity evolution from micron-sized dust grains to km-sized planetesimals: dust grains form extremely porous dust aggregates where the filling factor is ~10^{-4}, and then they are compressed by their collisions, the disk gas, and their self-gravity. The mass and porosity of the final product is consistent with the comets, which are believed to be the remnants of planetesimals. Furthermore, the radial drift problem is avoided due to the rapid growth of large cross section of dust aggregates. In addition, we have calculated the opacity evolution of porous dust aggregates and found that previous astronomical observations could also be explained by fluffy dust aggregates. We will also discuss possible ways to measure the porosity by polarization observations at millimeter wavelengths.