Milkyway is a Synopsys library format that stores all of circuit files from synthesis through place and route all the way to signoff. Most Synopsys tools can read and write in the Milkyway format including Design Compiler, IC Compiler, StarRCXT, Hercules, Jupiter, and PrimeTime. Milkyway is not a free tool. You have to purchase license for this from synopsys. https://en.wikipedia.org/wiki/EDA_database#Synopsys_Milkyway