All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
First of all, there is a difference of opinion among the scholars regarding a woman removing the hair that grows on her face. The most preponderant opinion –Allah knows best – is that it is permissible to remove it, because it is among the matters about which the Sharee’ah kept silent. Salman Al-Faarsi narrated that the Prophet said: "The lawful is what Allah has made lawful in His Book, and the forbidden is what Allah has made forbidden in His Book, and whatever Allah has kept silent about is forgiven.” [At-Tirmithi]
As for the hair of the eyebrows which grows and increases in density, then if it does not come out of the natural limits (original shape) in a way that it deforms the appearance, then it is not permissible for you to remove anything from it. However, if it come out of its natural limits, then it is permissible for you to take from it. However, this should only be to the extent that restores it to its natural limits and gets rid of the defect.
Allah knows best.