There is a very good website that you can visit, it provides many information about this "bubble" (CPC-Choroidplexuscyst ) . http://www.choroidplexuscyst.org
Actually, don't be over worry la! As normally, the bubble will be disappeared in late 2nd trimester - around 24-28weeks. And, it will not hurt or affect any of the baby's brain development.
So, apart from the bubble, did your doctor said any abnormal found from the baby, if no, then normally, everything will be fine.