We do not currently know of any antonyms for yuga.
The noun yuga is defined as:
In Hindu theology, an epoch or era within a cycle of four ages: the Satya Yuga (or Krita Yuga), the Dvapara Yuga, the Treta Yuga and finally the Kali Yuga, with lengths ranging from 432,000 to 1,728,000 years.