'This Is Us' season 1 spoilers: What on Earth happened to Jack?

There's so much going on in "This Is Us" season 1, but the biggest mystery is still the fate of Jack (Milo Ventimiglia), former husband of Rebecca (Mandy Moore) and father to Kevin (Justin Hartley), Kate (Chrissy Metz), and Randall (Sterling K. Brown).

In the second episode of "This Is Us," an older Rebecca was shown to have re-married, leaving fans to question how and why her marriage to Jack ended and whether or not he is still alive.

In "The Big Three," present-day Rebecca shows up at Randall's door with Miguel (Jon Huertas), Jack's best friend. Randall's daughters call the two "grandma" and "grandpa," giving away the sad fact that Rebecca and Jack are no longer married.

So far, the first few episodes of the NBC dramedy's freshman run has revealed that Rebecca and Jack's marriage was far from perfect and had its own share of ups and downs. Still, fans of the series have been saddened and disappointed to learn that their love story eventually comes to an end.

There are several popular theories on what happened to Jack, including that (1) he died, (2) he and Rebecca are divorced, and (3) he has Alzheimer's.

There are some clues pointing to the possibility that Jack has died. Because Rebecca is still wearing the necklace Jack gave her, it doesn't seem like they parted on bad terms. This could only mean that Jack somehow died and she wears the necklace to remember her late husband by.

A Reddit user postulated that Jack could have been a police officer and that he died in the line of duty. Another theory is that Jack, who has been struggling with alcoholism, could have succumbed to an illness caused or exacerbated by his drinking.

Alternatively, Jack could still be alive but could have long ago divorced Rebecca and abandoned the family. In its first four episodes, "This Is Us" has already shown viewers the couple's marital problems.

Perhaps the couple never got over their issues and eventually decided to part ways. Maybe Rebecca fell in love with Miguel or Jack fell in love with someone else. Maybe the fact that Rebecca kept her meeting with William (Ron Cephas Jones) a secret from everyone could have had something to do with her and Jack's eventual separation.

A third possibility is that Jack is still alive in the present day but that he has succumbed to Alzheimer's. An Entertainment Tonight staffer theorized that there may be a "The Notebook" aspect to Jack's story and that perhaps before the disease completely took his memories, Jack insisted that Rebecca move on with her life. She and Miguel could have turned to each other for comfort and eventually became a couple.

"This Is Us" season 1 airs Tuesdays at 9 p.m. EDT on NBC.