An indictment is issued by a grand jury when they are convinced, on the basis of evidence, that there is probable cause to believe that the person committed a crime. However, the indictment only has to state the charges, i.e. the grand jury's conclusion; it doesn't have to describe the evidence that led them to this conclusion.

In particular, the grand jury has subpoena powers to compel witnesses to testify under oath. But their testimony is sealed and must not be made public, except by the witness themself if they choose to do so.

A good source for learning more about the grand jury process is the Handbook for Federal Grand Jurors.

So, there must have been some evidence that he committed a crime, at least in the grand jury's view. But we, the public, don't get to see it at this time.

If the case goes to trial, evidence will be presented publicly at that time, though it won't necessarily be the same evidence that convinced the grand jury to indict.

An indictment is issued by a grand jury when they are convinced, on the basis of evidence presented to them by the government, that there is probable cause to believe that the person committed a crime. However, the indictment only has to state the charges, i.e. the grand jury's conclusion; it doesn't have to describe the evidence that led them to this conclusion.

In particular, the grand jury can hear the testimony of witnesses under oath, and has subpoena powers to compel testimony. But testimony before a grand jury is sealed and must not be revealed by anyone except the witness themself.

A good source for learning more about the grand jury process is the Handbook for Federal Grand Jurors. The authoritative source is the Federal Rules of Criminal Procedure, Title III.

So, there must have been some evidence that he committed a crime, at least in the grand jury's view. But we, the public, don't get to see it at this time.

If the case goes to trial, evidence will be presented publicly at that time, though it won't necessarily be the same evidence that convinced the grand jury to indict.

(Note that this answer is about the US federal criminal justice system, since that's what's involved in the Collins case. Some states may have a similar system; others may not.)