Angular 4 and 5:

Using else:

<div *ngIf="isValid;else other_content">
    content here ...
</div>

<ng-template #other_content>other content here...</ng-template>

You can also use then else:

<div *ngIf="isValid;then content else other_content">here is ignored</div>
<ng-template #content>content here...</ng-template>
<ng-template #other_content>other content here...</ng-template>

Or then alone:

<div *ngIf="isValid;then content"></div>
<ng-template #content>content here...</ng-template>

Demo:

Plunker

Details:

<ng-template>: is Angular’s own implementation of the <template> tag which is according to MDN:

The HTML <template> element is a mechanism for holding client-side
content that is not to be rendered when a page is loaded but may
subsequently be instantiated during runtime using JavaScript.