There are actually a couple of ways to go at this. First, completely putting aside you are his spouse, Illinois does not have misprision of felony, which is what you are describing. Second, if we assume the crime did not involve your children, you have an inter spousal privilege and cannot be compelled to testify about anything he told you. So, if you "know" because he told you, you enjoy a privilege not to tell. In other words. Illinois sympathizes with your position.
I'm sorry for the burden you must carry.
Generally speaking in Illinois, there is no duty to report a felony. Merely knowing about a crime and failing to report it is not a separate criminal act. However, depending on what steps you've taken to keep the crime a secret, you may be charged with aiding and abetting, obstruction of justice, or other related offenses. You should refrain from posting details of your case on the internet. These forums are public.
Keeping a secret means not talking about it at all - here online, in other public places, or even in private. Although you believe you have not said anything of detail here, your message could be identified to you if the proper authorities are monitoring. You are under no obligation to report the commission of a crime, particularly of a spouse. If you played no art in the action, then you probably would not also be charged. However, your post here is dangerous to both you and your husband. You should write to Avvo customer service and request that your question be removed. If you have further questions or concerns, you should speak in person and in private with an attorney where you can have a confidential conversation.
Although AVVO describes this site as providing free legal advice, it is really a simple Q&A forum. The volunteer attorneys provide general answers. No specific legal advice is given here and no attorney-client relationship is established. For precise direction and legal advice, please consult in person with an attorney in your area. Be sure to bring all relevant paperwork with you.