I had a similar feeling for a while and it turned out my DH had developed a severe porn addiction :(
The first time I suspected, and asked him, and he just lied to my face. So your fear that he will deny is valid.
The second time I asked, he told me the truth. Our sex life had basically disappeared so he knew I suspected it.
I hope for your sake it's just emotional distance that has arisen in your relationship which can be worked on with regular date nights etc. But it may be something more serious - perhaps porn, gambling, substances, or an emotional affair.
I suggest you investigate first, to avoid any potential lies and gaslighting, which can badly damage your mental health, self-esteem, the relationship, everything. Unfortunately, when they are in active addiction, they can become dishonest and selfish, even if that isn't who they really are.
It has been a lot of work to rebuilt trust but we are strong again. I had told him I wanted to divorce and gave him back my wedding ring (okay, I actually threw it at him, I was so angry and heartbroken), and it was only then that he realised how serious it was and that he could lose me over it. He went to a sex addiction therapist and did 12 step meetings, solidly, for two years.
The only reason I believe and trust that he is still 'clean', apart from our physical relationship getting back to how it was, is that he has managed to give up other addictions in the past. We're both ex problem drinkers, but I had a few relapses over the years, whereas once he decided he was stopping, he just did it and never went back. He was the same with smoking. So while he has had problems with addictive behaviours, he's also got a good track record of determination and hard work in stopping them.
Also, I don't have the gut feeling that anything is wrong. When he was engaging in the secret behaviour, it drove an emotional wedge between us, something that was almost unconscious, but I could feel it.