Matter= (1) a subject, situation or event under consideration: ‘It is a matter for the police.’ (2) used to mean ‘problem’ or trouble’ only in questions and negative sentences: ‘What is the matter?’ ‘There’s nothing the matter.’
It doesn’t matter + clause: ‘It doesn’t matter if you can’t answer all the questions. Just do your best.’
Subject + doesn’t matter: ‘The results don’t matter. Just do your best.’
Nothing/something is the matter or there is nothing/something the matter: ‘I think there’s something the matter with the central heating. It’s cold here.’ ‘Don’t worry. Nothing’s the matter. It’s just a tiny cut.’