不定式省略问题(…that most users do not even know exist)

TheUSlooks like a Wild West for personal data. Information gathered by companies — most prominently 谷歌 and Facebook — trades at lightning speed on advertising markets that most users do not even know exist.

原文第一段http://m.kekenet.com/read/201705/507695.shtml

这句话最后，exist前面是否少了不定式符号？

The US looks like a Wild West for personal data. Information gathered by companies — most prominently 谷歌 and Facebook — trades at lightning speed on advertising markets that most users do not even know exist.

【原翻译】就个人数据而言，美国看着像荒野西部。企业——主如果Google和Facebook——采集的信息以闪电般的速度在多数用户甚至不了解存在的广告市场上买卖。

1. exist 之前不可以加 to。

2. that most users do not even know exist：定语从句，修饰 advertising markets

3. 在定语从句中，most users do not even know 是插入语。

说明：插入语不是主要成分，但它是有语义的。虽然它跟定语从句的内容并不密切，但去掉后，它本身表达的语义就没了。目前，大家把它去掉，是这个样子：

4. advertising markets that exist：此时的that exist 是核心定语从句。

关于定语从句中的插入语：

有时，在定语从句的关系词与从句中的句子成分之间，插入类似：

I think/believe/guess/expect 与I am sure,they say，we know 等插入语，地方一般在关系词之后，以表明说话者对问题的怎么看、态度、评价、知道等内容。这种定语从句被叫做“混杂式定语从句”。比如：

1. Jocelyn is the only person who we expect will win. 乔斯林是大家期望唯一能胜的人。

2. Many college students have mastered computer, driving and foreign languages,which we all know are the most important things in todays society.

不少大学生都学会了计算机、驾驶和外语，大家都了解这类在当今社会是尤为重要的。