Is this a sign of definite cheating?

I'm new to the forum. But my name is Bradley. I'm 28 years old. I have been in a relationship with my girlfriend for almost a year now. Everything is going great. Then I was using her phone the other day because my died. And I seen a number to a guy named "Jonathan" it showed in recent calls, she had called him late at night for and hour at a time.So, here's what I did. I wrote down his number, and I texted him from my buddies phone (a phone number that my girl doesn't know ) and I told the guy (in the text) that his number was on my girls phone and he had called and texted her phone and I asked him why and who he was. I sent two different text. Two hours apart. And dude never ever replie back. And I know he got the text because it was from iPhone to iPhone apparently and it shows when they are read. He never replied back. So my question is, from another guys point of view why wouldn't he reply back at all? If it were me I'd at least say something back. But this guy didn't say anything and won't answery text.So, please don't be rude or leave smart comments about I should I have done this or shouldn't have done that. Just please answer the question I'm asking honestly. If not, then please no reply at all. My question is why wouldn't a guy answer a random text like that back? I'm a guy, but I don't know why a guy wouldn't answer back. So it has me wondering even more, if that's an even worse thing.

I think it's because he's afraid that you'll find out about them. You should definitely confront your girlfriend about this. I'm not sure if it's cheating or not, but maybe he's already alerted her of it. It's not normal for a women to stay up and constantly text some other guy unless something fishy is up. If I was you I'd be suspicious too.

Before reaching out to him you should have asked her about it first and got her side of the story. Then I would have sent a text from her phone if possible to see what kind of conversations they have. Or you should have simply called him (not text) and asked him what was going on. He could have answered your question or hung up. He would have been a fool to respond. He could have many women he's texting.

You are concentrating on the wrong thing. You need to talk to your girlfriend about these phone calls.
This has nothing to do with the guy not texting you back, this has to do with your girlfriend talking to some guy you don't know for an hour at a time!

This guy owes you nothing, not even the time of day. What's in it for him to get back to you? He doesn't know you, want to know you or give a rip about you. You are dead to him. Busy guys don't waste their time on unsolicited calls.

He could be anyone and anything. He could be a relative, a physician, her attorney, her CPA, anything. If professional he has a duty to not speak with you about her without her permission. (yes, I'm older and a professional).

and if your girlfriend finds out, she will be irritated with you and you may find yourself a former boyfriend for invading her privacy.

That's not really enough evidence. She could be cheating but I wouldn't jump to conclusions. If you find more solid evidence then I would confront her about it but for now just don't mention it.

Well Pearl, who goes by Bradley. Maybe the guy knows your girlfriend has a boyfriend and doesn't care. You aren't important to him and he has nothing to say to you. Or maybe he's thinking, "oh sh*t! The guy found out about us and he doesn't want you to know who he is or answer any of your questions about his relationship with your girlfriend.

Maybe you should use your girlfriend's phone and send him a message (pretending to be her) and say, "he knows about us. What are we going to do?" See if he answers that message. OR you could just confront your girlfriend about the guy she's cheating on you with.

You don't know WHO that guy is, it could be a friend, cuzin anything. You asumed yes its weird she called him late. But imaginig it something important you can look really controlling. And confront her or check her texts

He didn't answer for 2 reasons: he doesn't know you and he has no obligation to tell you anything. If you want to know who he is and why he texted your girlfriend, you need to ask her about it.