When couples split up most people think in terms of divorce, and indeed the vast majority of couples that do split up do indeed get divorced. There is however an alternative to divorce, which is nullity.
Nullity is very very rare indeed. Normally you can only get your marriage nullified if there was something obviously wrong with it in the first place.
For example, if you have got married, but you are not respectively a man and a woman your marriage will be nullified. If you got married and one of you is under the minimum age or you are related to each other within what the law calls the prohibited degrees, that marriage will be a nullity also.
There are also situations where a marriage can be a nullity by choice of the parties, as for example, with non-consummation or where the wife is pregnant by another man and didn't mention it to the husband. Or when one of the parties to the marriage was at the time suffering from some strange mental delirium which made them think that they wanted to marry the other.
All these can be legal grounds for nullity of marriage, and the difference between nullity and divorce is that if you get divorced that means that legally you used to be married, but you are not any more, whereas nullity means that legally you were never married in the first place.
See a lawyer and find out more if you think this might be of any relevance to you.