Perhaps some people have already thought of this. If not, this is an excellent place to share it. I am of the opinion that Happy Quinn's husband is actually Mark Collins himself! Now while this theory might be hard to swallow, it certainly would be a surprise for the show and would not be uncommon since there have been many big surprises in Scorpion already, such as the shocker finale ending itself when Happy reveals that she is already married and declines Toby's proposal to marry him. As to why and how this theory is possible, let's look at some evidence from Plutonium is Forever and Toby or Not Toby.
Mark's statements regarding Happy
In Plutonium is Forever, Happy wishes to run a full diagnostic of the power plant's computer system before starting the SCADA update. Mark points out that Happy's diagnostic could crash the system and prematurely melt down the power plant. During his arguments with Walter, he also mentions that Happy's opposition toward him is personal. The statement could and most likely means nothing; however, with the knowledge of this theory, one could ask how personal is it. Is it possible that Happy and Mark never get along because their marriage fell apart when he became trapped in his own mind and when Walter sent him away to an asylum?
In Toby or Not Toby, when Mark attempts to get into Toby's head, he finds the ring Toby planned to give to Happy in Toby's satchel. Looking back on the episode more closely, knowing how it ended, the conversation between Toby and Mark in this scene reveals something interesting. Mark says that he's known Happy longer than Toby has and that there is no way she would say yes to marrying him because she can't, implying that he already knows that she's married. Either he knows who she's married to, or he is the one that she is actually married to.
Happy's anger towards Mark
We all know that Happy has some anger issues, but she appears to be the most discouraged after meeting up with Mark again in Plutonium is Forever. In her argument with Walter after he chooses to not run a system diagnostic, she yells at him and tells him that if he takes "another trip down the rabbit hole" with Mark, that he would be on his own and that she would not go through it again. Again, while this most likely means nothing, is the fact that her anger issues are causing her to lash out at Walter or is it her relationship with Mark? Also, later in the episode, when Happy and Mark return to Mark's house to get the tape on how to get Cabe out of the reactor before he gets radiation poisoning, Walter sends Paige along with him, saying that her and Mark might not "play nice" with each other. While he may not realize that Happy and Mark are married, he did realize that she had more aggressive feelings toward him than anyone else on the team did.
In Toby or Not Toby, during one of his phone calls with Walter, Mark tells Happy to come on and watch her fiance die to the acid. This is when Happy realizes that Toby plans to propose to her, to which she yells at Mark and says that she's coming for him. Again, she appears to be more angry at him than anyone else on Team Scorpion. This could also be for many reasons, either because she is in love with Toby, but it could also be out of her hatred for Mark.
So, if this theory is true, the next question to ask is why Happy hasn't filed for a divorce. One explanation could be that Mark simply doesn't want one and wants to win Happy back. The other could be that Happy, despite her pessimism and anger issues, still has some feelings toward Mark and hopes that he will come to his senses and come back to her. Perhaps it is a combination of both of these theories and Mark has gotten into Happy's head, making her hope for him to come back, even though he may never do so. Despite the craziness of this theory, it is very plausible given the evidence from the two episodes Mark has appeared in, and I'm sure it won't be the last we see of him. What do you guys think of this crazy theory? I'd like to hear your thoughts.