I suspected the reason you haven't told anyone is because that would make it real and you'd have to act and your post at 1707 confirms this.
Understandable, but honestly once you do tell someone in real life it'll be a relief.
And people DO understand that what goes on behind closed doors is often very different to what happens in public.
Ever heard the phrase "street angel house devil"? It basically describes many abusers who are able to put on a pleasant, even charming front outside the house and when others are about but revert to their true devil side when there aren't others to witness their behaviour
My dad - a violent and otherwise abusive alcoholic behind closed doors, to others is the "life and soul of the party", friendly, charming, generous (always first to get a round in - meanwhile my mother's pinching pennies to buy food!), funny, intelligent... at home - grumpy, critical of EVERYTHING we did "wrong" but completely incapable of taking criticism, sulky, controlling, penny pinching, morose, creepy bastard!
It DOES happen gradually, if it didn't nobody would stay with them! It also serves to make their victim/s second guess themselves and not believe that's who he really is - the abuser as opposed to the friendly, charming man he PRETENDED to be early on in the relationship.
The first few times he hurt mum it was during "playfighting" and was laughed off.
It's also why key stages in escalation of abuse are when the victim becomes more tied and more vulnerable to an abuser - when you move in together, marry, get pregnant, give birth etc
The first time he "properly" hit her was the month after her being pregnant with me was discovered, this incident was minimised as "stress" over the pregnancy (unplanned) and impending (shotgun!) wedding.
The next time was a week after they married and from that point on it became a weekly thing, the cliche of him going to the pub on a Friday after work, getting pissed and coming home spoiling for a fight! Spending the Saturday hungover/sulking, then weeping apology on the Sunday.
But over the years it became twice a weekend, then 3 times a week etc.
As kids we learned NOT to be around when he came home on a Friday night and to be perfectly honest we came to dread hearing his key in the door at all!
I left home at 17 basically because home wasn't safe. If I could have left earlier I would have.
I suspect if you confide in someone close to you they will have noticed more than you think
Sometimes true sometimes not - I've found it depends on their own experiences, those who've had happy, safe homes and relationships tend to be less aware, those with some experience of similar are more likely to notice the nuances.
People I disclosed to once I was an adult, some of whom had been neighbours of ours at the time, were either completely blindsided or "I thought something was amiss", and what was interesting was I then discovered who of them had been through similar and so I was in their position kind of and most of the time I'd sussed correctly who had ok homes and who didn't though some surprised me.